Playing in time is one of the hardest things to master on the guitar in my opinion. A lot of times you don't even realize that you're not playing in time unless you record yourself and listen to the playback.

Playing with a metronome really helps. The biggest thing that has helped me is trying to think like a drummer. Practice your picking so that you can pick very evenly with 8th notes, 16 notes, etc. off of the beat marked off with your metronome. If you work on making your picking more even it seems to help you play in time better.

But playing with good time is also one of those weird things that seems to get better if you relax more. If you concentrate too hard on it then it tends to work against you.

-- Jim