When you pick a scab on a tattoo, it can take the ink with it and leave you with a an ugly inkless scar in its place. Depending on the severity of the damage, it may not be possible for your tattoo artist to fix it. If they can tattoo back over the scar, there's a good chance it still won't look as good as the original coloring. It's best to leave the scab alone and wait for it to fall off on its own, as difficult as that may be.
Caution After Washing or Showering
After taking a shower, you may notice that your scabs are very soft. They can soak up a lot of water and be easily torn, even just by applying lotion afterward or drying off a little too vigorously. Be very gentle on the scab until it's had time to dry - at least half an hour. Then you can apply lotion to your tattoo. Softening the scab with a small amount of lotion can reduce the pain from tugging against the skin, but you still don't want it to come off until it's ready.
Signs of Infection
Scabs can, unfortunately, be the birthplace of infection. If you can see pus oozing out from under the scab, if it is very tender and/or hot to the touch, or if there is redness surrounding the scab, you may be starting to get an infection. Applying hot compresses (as hot as you can stand) will help draw out pus. Adding sea salt to the hot compress can also help draw it up more effectively. After several minutes of hot compresses, apply an antibacterial ointment or spray - I actually recommend Bactine spray, which also contains lidocaine to ease pain and itching. If the infection doesn't improve in a day or two, see your doctor. At this point, your health is more important than preserving the look of the tattoo.