I use a bit of mild soapy water when fitting heli tape.
Put a few drops of baby bath in a small spray bottle, spray some onto the frame where the tape is going to go and then some onto the sticky side of the tape itself.
It gives you the chance to move the tape around and get it perfect.
Once on the frame, hold you finger down at one end so the glue sticks and then slowly run your fingers along the tape working out any water or air trapped underneath.
A good tip is to make sure you spray some of the water onto your fingers so you don’t leave finger prints on the sticky side of the tape.
EDIT – I’ve just heli taped a brand new frame using this method and it’s actually quite therapeutic 😀