I found this problem after changing cars to a people carrier thanks to the need to sometimes carry 3 kids, 2 of whom are gangly legged teenagers and a dog (also gangly legged) in the back.
Had no problem lifting bikes onto the roof of the previous car (a Focus) but really struggled with the people carrier (Nissan Tino). The solution I eventually settled on (after taking a step ladder with me in the boot) was one of these:
Cost me about £3. It folds flat and fits under the driver's seat when not in use. Gives me just enough extra height to lift the bike up without any hassle.