I usually shop rates on Orbitz then book direct with the individual company. Kayak.com is good too. So in June, I reserved a mini van from DIA with an 8 day est total rate of $450; just checked and the rate is $1230! Check back a few times every week. Rates supposedly change Tues/Weds and if you see what appears to be a bargain, grab it as it may not be there the next day. No out of money cost to reserve or cancel, but beware they can charge you if you want to make a change and may treat like a new reservation at prevailing rates. When you get close (2 to 4 weeks) of travel date, try Price Line to shop against what you have to see if you can do better, this service you must pay upfront.
INSURANCE, if you have an American Express card call to inquire about their sign-up for auto-rental coverage program. No cost up front but when you use the card to sign up for the auto, their coverage engages to become the primary carrier for $24.95 per rental period. Coverage also includes theft and loss of rental revenue due to collision / repairs with no deductables. https://www295.americanexpress.com/premium/car-rental-insurance-coverage/home.do?pcrid=17524142225&pmt=b&kw=premium%20rental%20car%20insurance&extlink=ps-cardserv-srNTt30Lk&gclid=CNvbkb-vw7MCFfBcMgodoFIA9g Doesn't hurt to call AmEx a day or so in advance just to confirm things.
When selecting an auto off the lot, and presuming you have a choice, try to find one with a cracked or damaged windshield and make certain the damage is noted on the contract when exiting. This way, any further damage to the windshield is waived. Lots of stones are tossed up along I-70.