I decided to post this because the 29 year old I work with just took a fall. There are tons of things on the market to help with this, from cheap little plastic stick-ons to expensive tubs that open so you can walk into them.
I'd like to suggest another option... simply lay a wash cloth on the bottom of your tub before you start the shower, step on the wash cloth and you will not slip. If you're more comfortable with a bigger area use a hand or dish towel. When you're done with your shower wring it out, hang it to dry and it's ready for tomorrow. Sometimes simple is best!