I have a microfiber seat cover that does the trick. I used towels prior, but there is no cotton terry thick enough to prevent my sweat from soaking through to the seat. I hang it up in the garage when I get home and run it through the washing machine every few months. I like that it's easy to pop on when I need it and just as easy to remove. I've been using it for the last 2 1/2 years, no complaints.

The real holy grail for me now is something similar to put between me and my seatbelt. I'm currently using several microfiber towels plus a length of fleece fabric, but it's bulky and uncomfortable and it shifts around and I end up with damp spots on the seatbelt anyway. I've looked at some seatbelt covers, but I don't want to have to leave them on all the time, nor do I want to have to line up 22+" of velcro to put them on every time I drive to a run.