I'm a complete fan of lensrentals.com. Only time I've rented elsewhere was a last-minute rental here in San Antonio (can't believe they took full retail deposits on very used flashes) and a steal of a deal for a day rental from Glazer's in Seattle.
My high-level advice is to either rent lens you honestly think are high on your "buy" list so you know what to buy, and/or rent lenses you know won't make it up your list anytime soon when they make sense to try. I started out owning a 24-105, and in one shot rented the 16-35/2.8I, 70-200/2.8IS, 100-400, and 1.4x. I went in thinking the 16-35 and 70-200 were high on the buy list, and came out knowing the 70-200 was to be purchased ASAP (the 16-35 came nine months later). I wasn't impressed with the 100-400, but maybe I'm too much of a fast-lens snob (I admit it!!!).
When in Seattle last August, with our 16-35 needing a visit to Canon repair, I rented the 14/2.8. I could have rented the 16-35/2.8II just like what we owned, but for $5 more I got to try the 14. It's fantastic, but I can't justify it anytime soon (my fiance wants it though...). I also tried the 200/2IS - holy cow, what an amazing portrait and action lens. For a lens I thought I'd try once (for $45!!!), it could be #3 on my lens-to-buy list. (So much for a lens I thought wouldn't be on my list anytime soon...)