Rent a Skin

At the champion select screen, where you can "Click to unlock" a skin, you should be given the option to rent that skin for just that game. Obviously, the IP cost has to be substantially higher than the anticipated IP gains for that game. I think a 1:1 ratio with RP would make sense (so a 975 RP skin costs 975 IP per game). You should also be able to rent the skin with RP. A good price point might be 100 RP for a 975 RP skin. Ideally, the RP rental cost would be refunded if you choose to buy the skin within one week of rental.

I think this idea would help Riot sell skins and provide an IP sink for people that own all the champs.