You could tie a spunge or some nylon on the intake of your filter. I don't know if these thing will change your water quality or not, someone else might know. AquaClear makes a spunge for inside filters that could be cut to the right shape. I also think there are products that you can buy to cover the intake and protect your fish but I am guessing lots of $$$.

Anita

Seems to me if a fish can't even stay out of the suction of the filter then that fish is on its way out anyway. Unless you've got some mondo oversized filter that would suck up a great dane. Is your filter sized to the size of your tank?

I know I'm late on this, but:
Actually, male bettas have a very hard time with filter flow, due to their long fins and the fact that they are not strong swimmers. NitaSher is quite correct, an AquaClear sponge or nylon (presoak to be sure it doesn't bleed color) are very good solutions for the intake. The sponge option will actually act as a pre-filter to catch large nasties before they can clog the filter (clean it regularly, of course). As for the outflow, it can be slowed by placing a suction-cup type soap dish (that has never seen soap, of course) under the outflow to slow the current entering the tank.
Good luck!

addition to the sponge pre-filter. this will also help colonize good nitrifying bacteria. when you clean it, use aquarium water in a bucket and squeeze. tap water will kill all the bacteria that you want to save.

if all you ahve is a betta, you could just use a sponge filter alone with an air pump connected. this should not create too much current. if you stay with a power filter, many have optional prefilters (sponge) made for them. some are interchangeable with other manufacturers power filters.

Our bettas always did this on purpose.

thats weird because i have never had that problem.maybe putting a sponge on it might help
good luck funky monkey

Thanks, I will try the Aqua Clear Filter. When he was sucked into the filter much of his fin was shredded, but it they seem to have repaired almost perfectly.
Thanks for the help!