What ways can help you strengthen your pelvic floor muscles?

Kegel exercises
are the best method. Link is a how-to.

Start riding horses!
The balance requirements and strength utilized by the pelvis works wonders.
People have no idea what a workout one really gets from riding.

Pelvis and upper body strength as well as great gluts and rock hard thigh muscles.

Buy a horse! LOL

Do what @janbb, then do as @Coloma mentions, or take up bike riding. My recumbent bike also works wonders for strengthening this area.

There are positions in yoga and in pilates that do wonders for the pelvic floor and the anus.

Kegels are fantastic! They help you when young to have great orgasms, feel snug with a partner and have an easier time being able to birth a baby. When older the Kegels keep you from needing adult diapers.

I do kegel exercise in and out of the shower. In the shower, once a week (no more or you’ll mess up the pH & good bacteria in the vagina) slowly let plain filtered water from a douche bag run into the vagina. When you notice you are full (the water will start leaking out) clench in a kegel exercise, and slowly stand in tub. Hold the water vaginally for a count of 60 then release the water and shoot it out. Do this twice more before you get out of bath.
Do this without water throughout the day, everyday, as much as you can. I also ride my bike a lot and in bad weather I put the bike on a trainer and ride while watching a movie. A good diet will do wonders too. Lots of fruit and greens.