While your sex life may have looked like the Kama Sutra when you were younger, some of those positions simply may not be possible when you're older. If you've got hip, back or knee problems, former favorites may have to be shelved.
But instead of getting frustrated, try to look at this as a chance to explore.
If one of you has arthritis, try something that puts the responsibility of movement more on the other person (hey, it's a workout!).
There's a reason spooning is so popular. Besides being great for intimacy when the two of you aren't having sex, it's also a pleasurable position for many people with hip problems.
Pillows are your friend! Try one between the knees, under the pelvis, under your back -- wherever you need to relieve pressure on a joint or specific area of the body.
It's all about communication -- and creativity.