Always practice with a metronome. It's great at working one your scales, rhythms and phrasing. Some say that practicing with a metronome is bad because it will become a crutch. You'll get so used to it being there, that you won't be able to keep a straight rhythm on your own. I disagree. Metronomes are very useful for getting your timing better, especially in the initial stages of learning. That said, it's important that you practice with a metronome but also incorporate other exercises to help with your timing. Also, always make an effort to play with other musicians. This will help your rhythm (and ears) immensely.
Play everything you practice at different tempos using the metronome. Most things are harder to play at slower tempos, not faster. If you're working on speed, this makes it easy to measure exactly where you are and how well you're doing. Don't get carried away with this though. Speed is nothing without phrasing, dynamics and feel. These are things we want to incorporate when practicing our rhythms. For example, don't just play through a scale over and over. Try dynamics on different notes and phrases. First, start of with accenting just one note (or chord) every bar. Start with 8ths and accent the first 8th note in each bar. Then accent the second 8th note etc. This really brings scales and phrases alive. It's something that we usually do automatically we strumming chords or copying solos but it helps when we break it down and do it on purpose. Next, try playing using different rhythmic patterns. Aim for controlled dynamics and smooth legato notes.
On The 2 and 4
When practicing with the metronome you'll want to try using it at different settings. For example, try setting the metronome at a slow tempo and pretend that that is the 1 and 3. Now practice your rhythms. This gets harder the slower you go. Of course most of our music uses the back beat so it's really useful to practice with the metronome on the 2 and 4. Also, try different rhythmic values like 3 on 4. Quarter note triplets and 5 notes to the beat are also interesting things to try.
On Your Own
Now try your rhythms without a metronome (or drum beat). This is something I usually don't have to tell people since it's something that most musicians do all the time. The difference here is you really want you to focus on your rhythm, That means just playing a basic rhythm or phrase over and over. No variation, no jamming. What you're trying to do is get your timing as solid as possible by just focusing on that and not on what chord or note to play next. For this exercise it's best to actually start with a metronome because we'll use that to keep track of our tempos. Start of with a very basic rhythm at a slow tempo. Start your metronome at the slow tempo to gauge the speed. Now, turn off the metronome and start playing the rhythm. Focus on keeping the tempo. Feel it in your head. Don't force it because that will make you want to speed up. Play the rhythm for a while, then go back and check your tempo on the metronome. How did you do? Yours won't be exact but you can gauge how fast or slow you were compared to the original. Try this at different speeds. It usually helps if you actually hear and play the rhythm in your head first, before you touch the instrument. Always take half a second to internalize the speed and rhythm. Record your practice and see how it feels on playback.
One of the great things I love about sequencers is how many ways you can come up with (and twist) loops and grooves. If you've used any sequencers you'll know about quantization. This effectively lets you control the amount of feel on any drum beat you have. :Let's look at a couple of ways you can use this to tighten up your timing.
As we talked about in the effective practicing post, it's a good idea to practice your scales (and chords, songs, etc) to a drum beat. By setting up a basic drum beat, you can play along and practice getting a good feel. What you want to start out with is a basic swing beat. I usually start with the bare minimum: the kick is on the 1 and 3, the snare is on the 2 and 4, and the ride is doing (strict*) swing 8ths. Using this basic beat makes me focus on the swing 8th note. I then start at a slow tempo and go through the various exercises. Start with scales, using different rhythm variations. Then try various licks and phrases. Got through some chord progressions, keeping the rhythm relatively easy, focusing on placing the chords at the exact place you want. Some sequencers allow you to vary the amount of swing. Again, set up a basic beat like the one listed above. This time though, sequence in (strict) straight 8th notes. Again go through the exercises we talked about: scales, chords, licks. Now, go back and try varying the amount of swing. Try 25% and see how it feels. Before playing a note, stop and really listed to the beat. Notice the difference between that and the straight one. Don't skip this step, it's really important. Once you stop and start really taking notice of the variations in rhythms, your ear will become sensitive to hearing these things.
*That means I go in and manually enter the groove.Yes it's mechanical and boring but for our purposes here, it's what we want.
Playing It Straight
Another thing that's great to do with sequencers is practice your rhythms with a straight beat. Set up a basic beat with no humanizing or variations. It helps our exercise if the beat is straight and boring. Again it's just a basic beat with the hi-hats doing 8ths. Now we're going to play a strict 8th rhythm and record it. Listen and lock in with the hi-hats. Once you've recorded your take, go back in and listen to your performance. First, listen to your track with the beat. How did you do? Is it in the pocket or does it go in and out? The best way to tell is to edit your take. Zoom in until you can see your rhythm track against the time-line. Do the transients of your rhythm track line up with the beats on the time-line? You'll find that most of the time you're either constantly early or constantly late. Most people are early, especially with slower tempos. Now go back to your track and move it back and forward a 64th. Does it sound better or worst. Were you early, late or right on. Fix the timing of your track until it's almost perfectly straight. Now listen to the track. If you can, compare that track to your initial take. Always listen back and take note. This is how you'll get better.
With the Band
Like I mentioned earlier, it really important that all musicians practice with other musicians. I can't stress this enough. I can't tell you how many times I've met musicians who can play the snot out of any scale and not have any feel at all. I've always found that musicians who had the best time, were the ones with the most experience playing with other musicians. When you do get a chance to get together with other musicians, take the time to practice just getting the groove. You'll find that grooves need a bit of settling. You'll start playing a groove and after a little time, it will just seem to lock (hopefully). This comes from settling into the groove, relaxing, and not worrying about what chord (or note) comes next. It's important that the rhythm section just works on the basic groove. No extras, no solos, no vocals. Just play the groove over and over. Work on listening to each other. Listen to each other and each part of the drummer's kit: listen to the hi-hats, then the kick, then the snare. Try to match what the drummer is doing. Great grooves come from knowing your instrument, your parts and listening to each other.
Doing It By Feel
After you've been working on your rhythms for a while, you'll just try to settle in and get the feel without thinking too much. This is great when you're playing with other musicians, recording, or just having fun. But you also want to dissect rhythms, practice variations, and incorporate new things into your playing. Try and incorporate some rhythm exercises into every practice session. While sometimes it may feel like you're not getting anywhere, these rhythm exercises will start creeping into your playing. You may notice that your playing gets better, and feels better. Always remember; the rhythm is paramount.