Degradation happens when the drive is completely full, and new writes have to erase old date first. The drive can fill up even if the OS doesn't show it as full, since stuff you delete doesn't actually get erased (unless you have TRIM support). Once every block is full, the speed will be constant for the rest of the life of the drive, unless you reformat/wipe the drive to restore the full speed (sometimes this takes a special utility, although a pass of zeros should do it). If you keep the SSD somewhat full and you are frequently writing and rewriting large files (video, disk images, etc), it will not be very long before the SSD "degrades." After that, speed should be stable no matter how much you write to it.
I don't know how much drives slow down when they "degrade." Look for benchmarks that compare a "new" drive to a drive with simulated use. My guess is 10-20% slower for writes and little change for reads, but of course that will depend on the drive. Even with that, the SSD should still be faster than any mechanical disk.
I'd say just get a SSD, use it, and don't worry about it. Don't worry about turning off indexing or deleting the sleep image (unless you need the space), and don't worry about what you are writing to it or the number of write cycles. In a few years they will be bigger, faster, and cheaper, and you will probably want to upgrade again anyway.