I try to smile as much as possible without it looking completely stupid or fake.
If I'm having a good ride normally I can manage to hold at least a little smile the whole entire time. But Dallas can be a tricky horse to ride, not bad, just tricky. He's very sensitive to what I do, or am feeling. So on classes I'm just trying to get through I often forget to hold the smile and go into total concentration mood.
Smiling definitely helps the judge's impression of you. I was at a show and entered Xrails with my 21(then 19) year old MorganXQH pony and we were against 25 other competitors. Shaymus is a great little pony and tries his best but he's all jumped out. He was once a children's equitation over fences pony, so now all he does for Xrails is step over them. But hey, I wanted to jump and Dallas was being a twitch so I entered Shaymus for laughs. Well he stepped over every single jump with a sweet expression, and I was grinning the whole time. Genuinely grinning, I was having so much fun I nearly burst out laughing on the course. I didn't expect a ribbon, but much to my surprise we got 2nd place. The girl who won had like the perfect round while smiling, and her horse actually jumped. Lol. The judge came up to me and said, "Congratulations. Your equitation was very good but what got you that ribbon was the fact that you and your pony were having more fun than anyone else out there and that counts for something." We both started laughing.
So try to smile as much as you can, but it's always better when the smile is real.