I have to do this every two years or so, which really isn't too bad. I have this "thing" about tub caulk being clean and non-mildew or moldy. Once it gets in there, it must be replaced or it just keeps growing. So, I've gotten pretty good at it by now! Today was the day I'd had enough, it got redone.
Here's my very easy way to do it, probably obvious, but here goes anyway.
Step 1: Remove all the old caulk, using a razor blade to scrape every last piece off the tub and/or surround. This is not fun, but necessary. I use a bit of Goof Off on a cotton swab when I'm done to be sure it's all off.
Step 2: Use painters tape to outline your area you are going to caulk. This is great if you don't want to try to be "perfect" without it. It comes out perfect every time this way.
Step 3: Use a toothepaste crimper on your caulk tube, it eliminates the chance for air pockets.
Step 4: Apply the caulk according to the instructions on the tube. I apply quite liberally, as I know the tape will protect the area.
Step 5: Using a wet finger run from one end to the other, letting excess caulk go right on the painters tape.
Step 6: Immediately remove the painters tape and throw it away. You're done! Comes out perfect every time.