I took the rear wheel off my bike and when I replaced it the chain was slipping - How can I fix this?

I took the back wheel off to fix a puncture and when I replaced it the chain kept getting stuck as it went around - it didn't look to me like it was actually stuck on anything in particular. I took the wheel back off and shifted the gears down so the d�railleur should place the chain on the smallest cog, to make it easier to put the wheel back on. Now the chain goes around but feels like it 'slips' a little as it goes around... any ideas anyone?