I'd say no dogs, or cats. They can absolutely destroy your ability to rent it out quickly when a tenant's problem pet becomes a...problem.
Check local laws.
Your best bet, (since contracts don't really protect you, they just spell out the rules, which people can go ahead and break):
Get a great tenant. Be patient. An empty month while getting a good tenant is cheaper than having someone tie it up with an eviction or no payment for months on end. Get as much deposit as is legal in your area. Listen to the nerves in your gut and chest when you meet them. Don't call their previous/current landlord--he may want to get rid of them--call the one before that. Charge them for credit report along with a rental application, lying means the lease will be terminated.
But credit score isn't the biggest factor, rather, ask to see their budget. See how much debt they have. Do they have to commute far to their job? Do they have a huge debt load? Do they have a criminal history?
Basic leases are easily found. Some clauses to consider: No adult roommates without them being on the lease. No visitors longer than 2 weeks without your written permission. Must maintain yard, must shovel snow according to local laws. Smoking is grounds for termination.