The best tip I can give you is keep attacking.
If you are in side control or mount and they are tucking their arms in close to the body with elbows in, start working some chokes. Two good ones that I can think of right off the top of my head are North/South and Ezekial.
If you have side control and he's tucking those arms in go for the North/South choke that should be open if he has those arms tucked in. Depending on if the person is really active on the bottom you could "sucker them" into a sub. One that I like to use all the time is suckering people for a d'arce choke. If I have a nice side control on them and they are stuck with arms in tight not giving me much to work with I'll ease up the pressure on them...I'll actually give them enough room to make a shrimp. When they shrimp you have to instantly drop you back side arm (arm closest to your opponents waist) under their topside armpit and around the neck. You have to remember to drop your shoulder down to get the clearance out the other side. Once you have the hand out the other side all you have to do is lock up a figure 4 around the arm and head, tuck them in super tight, and flex all those muscles squeezing in tight for the finish.
If you are in mount I like working the Ezekial most times given the situation you are in. I have long arms though and it doesn't work for everyone nogi...some people just don't have long enough arms to be really successful with it. If I can't get the Ezekial I might move into an S-mount and start working them onto their side for an armbar...possibly working the leg over on one side for a triangle...really depends on what the opponent is giving me. On mount a good way I have found to get them to release those hands and move them is get real good at grapevining people...a grapevine can make mount extremely uncomfortable for the guy on the bottom and force them to move and open stuff up. Lots of times a person who gets caught with a good grapevine might reach up to push you off and relieve some pressure...most seasoned guys won't do this, but you can catch a lot of lower ranking guys like that.
If you just want to get the arms loose the best technique I think I have used in the past was getting a grapevine, then put both your arms under their arms (the interior of your elbow should be right on the inside of their elbow on each side), use your grapevine to stretch them out while shooting your arms straight up, and when their arms go up immediately slide into high mount.
Anyhow, there are tons of techniques, subs, and sneaky little tricks you could use, these are just a few that I like using on people.
Let us know if this helps any.