One tip I gave a while ago....here's the blog post on RUINED BRUSHES USING MASKING FLUID - click the link.
But another question arises when actually putting masking fluid onto the paper and what to use to apply it (other than brushes). The answer is, almost anything goes. If you find the edge of an old store/credit card works, then use it. The same goes for cotton buds, cocktail sticks. If you need to cover an area in preparation for sparkly water reflections, then an old toothbrush with masking fluid on it works well too. Just make sure you leave it to dry completely before continuing with your painting, that rule always applies when using masking fluid.
How to actually apply it....You can put it straight onto dry paper. But one tip I like is for SOFT EDGES and that isn't so difficult with masking fluid. Just wet the paper with clear water beforehand...not Just the area you want to be left clear of paint either...you need to apply the water further than you intend....add the Masking fluid and wait for it all to dry....when you have finished the painting and remove the masking, you will have a soft edge, the clear water will have done its job.
Sorry I have no pictures of this. I'm out of masking fluid at the moment!!! When I have some more, I will do a picture blog post to show the results of soft edges using masking, but hope this helps in the meantime.