If and when you decide that a lens deserves a permanent place in your camera bag, be sure to check the used market before purchasing the lens new. In my experience, most DSLR owners take very good care of their gear and the used lenses I have come across have mostly been in excellent condition.
I have purchased numerous lenses from local sellers on Craigslist and have friends with positive experiences from eBay. The key to buying from these sites is to know what to look for. Try to find sellers that have all the original packaging and provide clear pictures of the items for sale. If you meet a seller from Craigslist in person, bring your camera to test the lens and check the inside of the lens for mold and dust.
Other options for buying used include online retailers such as KEH, B&H, and Adorama. I have no experience with these retailers but know they are all reputable sellers of used photo equipment.