Finishing an armbar is a fantastic feeling, but if you can't hold the person in the armbar finish position -- the position where you have your legs across their body and head, but haven't yet broken their grip -- you can't finish a tenth as many armbars as you might be able to. Similar in nature to guard maintenance, the armbar finish position requires practice, but it's well worth the time you put in, as you'll use it all the time when rolling. Here are some fundamental concepts and tricks that will help you maintain this valuable finishing position on ever more partners.
Here's a quick concept that might clear up some of the confusion around a common misconception in jiu jitsu: crossing your ankles is okay whenever you're just maintaining the position, but you generally want to squeeze your knees together whenever finishing the armlock, which probably means not crossing them, or at least crossing your toes only. Isolating the far arm is another extremely important concept for controlling the position; if your partner's far forearm can sneak under your feet, you can be facing a serious escape threat. Meanwhile, pulling their far elbow in serves to prevent this escape almost entirely.If you're dealing with a partner who is typically able to come up on top during your armbar attempts, stacking you and then escaping, you can stop them from just coming up by flatting your back on the mat. Imagine that your back makes up a wheel if you're curled up. Now imagine stretching your arms out and making your back like an ironing board, and imagine further how much more difficult it must be for someone to come up on top.
Another helpful concept is that you generally want to stay "square" with your partner during the grip break portion itself, with your hips facing the ceiling, essentially. However, things change rapidly once you break the grip, particularly if there's an opportunity for your partner to rotate their thumb toward your head. The solution: put your right ear on the mat (if you're armbarring their right arm). This will enable you to fall toward their hips, facing their legs, and making any kind of technical escape all but impossible.
There are typically two hand positions commonly used during the armbar finish position: the Kimura grip, and the "hand in the pocket" position. The "hand in the pocket" version allows two useful advantages: you can grip your partner's pants in order to help stabilize and maintain the position, much like with a bow and arrow choke; and you can post on the mat, ultimately preempting your partner's attempt to stack you, and allowing you not to have to stretch your arms out and flatten your back. Note: flattening your back works great to prevent the stack, but it's a bit of a late-stage defense to this, and necessarily means you have to sit back up and sort of reset the position; the preemptive option shown here (simply posting) requires much less energy. Of course, the Kimura grip may also prove useful, so figuring out how to maintain the position with both grips is a must.
Imagine being really, really good at something, but never having the opportunity to execute whatever that thing is. That's what the armbar finish position is without the maintenance aspect of it. Start viewing this as a position, just like side control or mount. This means that control has to come first (remember the ancient mantra of "position before submission"; this certainly applies to micro-positions, or positions inside of more orthodox positions). Focus on controlling your partner here, and don't even go for the actual submission right away, instead paying attention to how they're trying to escape so you can take mental notes and properly address the issues as they arise individually. As always, please let me know if these techniques are working well for you!