How to make bathroom tiles non slippery
A few products on the market (depending on the type of tile). They are sealers that when applied properly, etch the face of the tile and add skid resistance when wet. They do not change the appearance of the tile at all (as always, test an inconspicuous area and follow all instructions). Check local tile distributors,the products are generally very reasonable in price.

One is by Miracle, "511 non-skid", a good product. Another I've used is Invisa Tread, not sure the manufacturer. Miracle is a reputable sealer company and I think it runs about 15$ a gallon (here).

Be careful around unfinished wood or metal, pull the strainer off your shower as well. Good Luck