No matter how much time you have to practice, break each session into three parts: the Warm Up, the Hard Stuff (no, not that Hard Stuff), and the Fun Stuff. Always end with the Fun Stuff, so you’re not frustrated at the end of the practice session!
If you’ve got an hour to play, warm up for 15-20 minutes, play new tunes or hard parts of current tunes, try playing songs you know well at faster tempos for 15-20 minutes, then play fun stuff for the rest of your time. Even 15-20 minutes of playing EVERY DAY OR TWO is better than 3-4 hours once a week. Thirty minutes or an hour every day or two is great for most folks. Ideas for the Warm Up section: Chromatic scales in Two Octaves / Major-Minor Scales / Arpeggios / Chord voicings / Minor chords. If you’re a singer, warm up your voice with long, low tones.
Hard Stuff section: Use the Metronome / Building up speed / Play along with Slow Downer tracks or CDs at Faster Tempos / Playing at 40-60 bpm / New Songs / Challenging sections of existing songs / Practice with a capo / Kick Offs / Turnarounds / Intros. Play difficult sections backwards / Play 2-3 measures at a time, starting in the MIDDLE of a song. / Playing at 140-160 bpm.
Fun Stuff section: play songs you know well, or really enjoy playing / “noodling” / playing along with Slow Downer tracks http://www.ronimusic.com or with CDs at Slower Tempos. If you’re a singer, try singing songs in different keys to find your LOUD voice. If you intend to join in a jam session, always practice playing your instrument QUIETLY and also LOUDLY. If you intend to join in a jam session, always practice playing your instrument capo’d to common Bluegrass keys, like A, Bd, B, C and D. If your instrument doesn’t use a capo, practice playing scales and simple melodies in these keys.