1. Establish whether the horse is to be ridden exclusively in your tack or if the leaser can use their own. A poorly fitting saddle can cause problems quickly
2. Be clear on where and what the person can do with the horse. "No jumps over 2". "Horse not to be taken off property without permission", etc.
3. Agree on who is going to pay for what. With most 1/2 leases, the leaser pays 1/2 the shoeing costs. Owner picks up routine vet bills, shot etc. Figure out in advance who's going to pay if your horse comes up lame and also if your leaser is obligated to continue paying if you horse in unrideable for a longer period.
I definitely would watch the person ride your horse before agreeing to lease to them. If they ride a few levels below you, your horse can be quickly untrained and pick up bad habits. I'm not trying to scare you away from leasing, but just be very selective on who rides your horse. There are plenty of good leasers out there, so don't settle for someone who's going to mess up your horse.