Give your biceps priority during your workout. It would be best to work your biceps directly after a rest day, when you're fully rested and ready to give it all you've got. So if you're doing biceps and back on the same day, and your biceps are lagging, then you might want to try working your biceps first. Your biceps are going to be tired once you start your back routine, so you're not going to be able to lift as much weight for your back exercises, and therefore your going to be sacrificing somewhat in that area. However, if you really feel like your biceps are lagging, then giving them priority is one of the best ways to fix that. I'm assuming you've got a good foundation of mass and are now wanting to fine tune. Otherwise, I'd keep concentrating on building mass and wait to bring up lagging muscles later on.
Another tip when it comes to working the biceps is to concentrate on having great form. Even if you have to reduce the amount of weight you're using, try to maintain the best form you have. Remember, you don't want to make the exericse easier by cheating and calling in other muscles, you want to make the exercise harder by making the biceps do most of the work. Concentrate on full range of motion, contracting the biceps at the peak of the movement, then slowly lowering the weight all the way. Try to keep tension on the biceps at all time. Sometimes you'll see people doing curls and they'll throw the weight up to their shoulders, lean back a little, and try to rest, when they should be keeping tension on the biceps constantly, from the peak of the movement, all the way to the bottom. Lately I've been using an arm blaster device on my barbell curls and I can really concentrate on great form and making the biceps do all of the work.
Lastly, try flexing and stretching your biceps between sets. After you get done with a curl movement, try pronating and supinating your wrist as far as they'll go in both directions. The function of the biceps is to flex the arm and help rotate the wrist, so remember this when you're training them. Flexing and stretching between sets not only keeps your pump going strong so you can bring the most intensity to the next set, but it also helps with flexability, and over time, actually helps to stretch the muscles somewhat. When you stretch the musles over time, you're creating more volume. Volume and mass are related in that if there's more volume, then it's possible for more mass to fill that volume. So the more your muscles stretch, the more mass that can occupy that volume. Now I'm not saying you'll be able to drastically change the shape of your biceps, but I've been doing this for a while and I've been noticing I can contract my biceps muscle harder, and they appear more full. I didn't make this up either. Lots of bodybuilders do this and swear by it.
Hope that helped some!
