Leasing for me was a great experience. While growing up I didn't have my own horse so I was leasing horses for shows and the county fair. I would always make sure that you and the horse owner communicate well with each other and make sure the horse and you have a connection. You want to make sure everything that is expected of you is in the lease paperwork and that you have discussed what you want and what is expected of you while leasing!
Really make sure that all the rules are clear right from the beginning as to what you are allowed to do with the horse, like jumping it or taking it off property. And who pays what is clear. As well build the best relationship with the owner of the horse and make them feel confident that you are taking good care of their horse.