I had my first trumpet lesson in Mexico about a year ago from a jazz musician we met on holiday. He was on holiday with his band also, their gigs paying for the trip so everyone was having a good time. He showed me a couple of "lower face strengthening exercises", and since I haven't seen them in here, I thought I'd pass them along.
1. Face Crunches .... This is what I call it, the lesson was in Spanish so I really don't know what he was saying, but it's easy to demonstrate. Simply tense up the muscles around the mouth, into a tight lipped grimace, and hold, release, hold, release, etc. just like you're doing situps. If you try this for a minute (or less), you should feel it.
2. Tongue Circles ... Run your tongue over your top teeth and continue the circle over your bottom teeth (either in front or behind) with your mouth closed. It's harder than it sounds.
3. Fish Pops .... I don't know what else to call this one ..... Open your mouth slowly and then close it very quickly, which should result in a little pop (kind of like you get with valves) ..... That'll strengthen your jaw.
The last one involves an empty (Mexican) beer can and a shoelace but I can't recommend this one as it might be hard on your teeth.
Give em a try ... I can't really comment on how often this is recommended, or how long or any other details, like I said, it was in Spanish. He did say "Train like an athlete" (someone translated that...).