Riding with your cinch too loose can be a big problem for you, causing your saddle to slip; while fastening the cinch too tightly can cause a big problem for your horse by making him sore or giving him girth galls or even a hemotoma. Here are a few tips to make sure both you and your horse are safe and comfortable for the duration of your ride.
Make sure you take your time to tighten the cinch/girth and don't gut-wrench your horse as soon as you saddle him. When you first put the saddle on, snug up the cinch just enough to hold the saddle in place while you finish getting ready for your ride. Take your time, tightening it at least three or four times before you mount and walking him a few steps before tightening it enough to get on. Don't over do it! Your cinch only needs to be tight enough for you to mount without the saddle slipping. If your horse has good withers, you won't have to over-tighten; if he has poor withers, he'll have to get used to a much tighter cinch.
It's a good idea to check your cinch for tightness again about 10 minutes into your ride. Your horse does not blow-up his belly to make the cinch looser later on; horses don't think in the future. However, after he's warmed up, the pad and saddle have compressed and the air is out of his hair coat, the girth may be a little loose. Check again for tightness after you've warmed up and before doing any canter work.
Most people check for the tightness of the girth just above the horse's elbow, but that may give you a false reading, since most horses are concave there so the girth may feel loose even when it is tight. To really know for sure how tight your cinch is, you must feel it where the cinch crosses bone at the horse's sternum (between his front legs). Insert a finger from the back forward (so that when you pull out your finger, the hair smoothes down); you should be able to insert one finger to the first joint. If you cannot, it is too tight and if you can get more than one finger in, your girth may be too loose.
If your horse is "cinchy" or reacts negatively to tightening of the girth, he probably has good reason; someone has probably hurt him with the cinch at some time. It may have happened the very first time he was saddled or some time later in his career, but hurting or scaring a horse with the cinch can make him resentful about saddling.