I've rented probably 8 or 10 times over the last 5 years. It's a great way to try different bikes and see parts of the country you probably wouldn't ever visit on your own bike.
1. Read the dealer's website carefully and call to confirm everything or ask questions. Sometimes a day is from that dealer's opening to closing. Sometimes it's actually 24 hours.
2. Double check what's included, like helmets and rain gear.
3. Check with your insurance company about coverage on a rental bike. Mine only covers rentals if your primary bike is in for repairs or service, so I always opted for all of the additional coverages offered by the dealer.
4. Visit AAA for some good maps of the area you're going to visit or have a GPS or phone with that capability along.
5. Most dealers offer a HOG discount so be sure to bring your card if you're a member.
6. Don't be embarrassed to spend some extra time in the parking lot getting to know the bike. It might be very different from your everyday ride so don't just hop on and roar out of the dealer's lot.