When practicing rhythm stuff, use a metronome on either beats 1 and 3 or 2 and 4. Using a metronome on all beats makes your timing weaker IMO, as it spoils your "inner clock". When practicing fretboard knowledge stuff, do not use a metronome at all. Try playing arpeggios instead of chromatic exercises for warmup, it warms your fingers up as well as improving muscle memory on arpeggios.
I respectfully disagree with anyone suggesting a gripmaster or anything of that nature - you want dexterity, not physical strength. If you want to build up muscles work on your back, it is of vital importance for bass players to have a strong and healthy posture. Bass is quite an instrument
I would suggest muscle stretching before a practice session, from the shoulderblades to the fingers. Be careful not to overdo it though, it's not that difficult to hurt a muscle, tendon or ligament.
Minimize noodling to a minimum and before you set down the bass after a practice session, play something you like. Remind yourself why you picked the bass in the first place
I hope I was helpful.
Oh and Glenn Letsch's Stuff! Good Things Bass Players Should Know is a very nice book on practicing and generally tips on how to get better.