I agree with BP's excellent advice. He did not address descents however so here is my 02 cents
When cruising at the higher altitudes (eg above 5000 feet) Never go to full rich before you start your descent. The rush of extra fuel will cause the cylinder head temps to rapidly drop which is a recipe for cracked cylinders.
If you are doing an A to B trip and need to descend as you near your destination, first plan a 500 foot min descent. The easiest way to figure this out is start down when your time to destination is twice the vertical distance (eg if you have 5000 feet to lose, start down 10 mins from the destination). This is dead easy if you have a GPS which is IMO a must have item for any smart pilot.
When it is time to go down simply pitch the nose down and trim for a 500 ft/min descent. The airspeed will increase which will help make up for what you lost on the climb. Leave the mixture where it is or if you want slightly enrichen it. As the airplane descends the RPM will increase so you will have to reduce the throttle periodically to maintain your cruise RPM.