This is one of those expenses I really have a hard time getting around. Sometimes, you just need a sitter. The best things I have done to really cut down cost in this area is to babysit for someone else for free (like a co-op) and then that person keeps your kids for free when you need them. Another option is family, just don't wear them out.
The final option is to plan your outing really well. I pay my sitters by the hour, and I round to the half hour. So, I plan what can be done in that time to maximize my babysitting dollar. If I have a MD appointment, I stop by the grocery store, bank, post office, etc on my way to and from the appointment. I also plan to arrive at my appointment on time, not 15 minutes early, 5 maybe, but not 15 because you end up sitting longer. While I am waiting I write any lists, take note of any ideas or thoughts, read or knit. Keep the idle hands busy. Do as much as possible and be as productive as possible when you are paying someone to sit your kids so that you can get the most out of your dollar and make your minutes count!