Always remove old caulk. The ProTouch caulk removal tool was designed by users to be the best caulk removal tool available.
Clean the surface to be caulked so that the surface is free of old caulk, dirt, soap scum, mold, mildew, and other contaminants. Use bleach or a chemical-based cleaner, but be sure to rinse off all traces of the cleaner before caulking.
Select the ProTouch nozzle size that corresponds with the size of the gap you are caulking. With narrow gaps, a smaller bead will look better and perform as well as a larger, less visually appealing bead.
Fill your bathtub halfway will water prior to caulking. This will flex the tub and open the gaps. This will put less strain on the applied caulk in the future so that you get long-lasting results.