Years ago, my DH made a couple of ramps and carpeted them. These seem to work the best as they can go to any height needed. I also have a set of Doggy Steps (
Doggy Steps 3 Steps - Now Only $19.95 - The Official Site of AsSeenOnTV.com) that work well too. They are good up to 70 lbs. I have seen some like these that have a much cheaper plastic framework under the fleece. Not good! This brand has a fairly sturdy frame. My grandson gives them a work out from time to time and they are still holding up well. They are inexpensive too.
There is a pic of my new baby girl using the Doggy Steps in my photo album in my profile here.