On guard.

Not like in the old swashbuckler movies, but in that this is a post on "the guard position" which I hate as a term. Jiu jitsu players tend to think of guard as a box that they put their opponent in. A trap, or prison to be escaped. We would be better served to think of our jiu jitsu guard the way a boxer thinks of having one's 'guard up' to maintain distance and defend against the opponent's offense. This understanding gives a boxer flexibility in how their guard presents itself; it doesn't have to look a certain way as long as it maintains distance, and keeps the other person from punching you (conversely just because you have your hands held in a certain position doesn't mean you're going to have an effective guard the proof is in the punching). It allows for differences in style. Muhammad Ali had very low guard because he had amazing head movement and fought very upright. Mike Tyson had to use a much higher guard because he was a much shorter fighter and needed to deflect shots on the way in. Styles make fights, but styles make guards too.
Building our guard we need to understand the fundamental purpose of guard play; to control pressure and distance from your opponent. Players usually get too caught up in methodology: control points (grips), movement, and frames. Instead of thinking of our opponent "escaping our guard" and frantically trying to cram them back in the box we can understand that if we deflect their pressure, or move out from under it, create distance (if needed) we can get our guard back up. Note I did not say "recover guard" your guard was not stolen from you, it was broken.. it is generally easier to repair something than to replace it.
Controlling pressure from your opponent is key to preventing a pass. A guard passer needs to keep constant pressure on your center mass to keep the guard player from moving (movement is key for an offensive guard). They need to keep you in place or moving in a single direction to rotate around and get into a dominant position. Controlling this pressure can be as simple as using a foot on the hip or bicep to stop the pressure coming forward, or can be something that deflects the pressure of your opponent. Either to get them moving forward or keeps them moving as the guard player moves out from under the pressure (arm drag from sitting guard is a great example of this, but also the hip bump sweep is the same concept in the opposite direction).
Attacking from guard requires movement, but it also requires the passer to be within a certain distance. Too close and the guard player cannot move, too far and they cannot connect to the opponent in any meaningful way. The passer wants to move through this danger zone as quickly (read efficiently) as possible. The guard player wants the passer moving back and forth through this zone until a sweep or submission changes the position or ends the match respectively. There are three ways to control distance: move yourself, move your opponent, convince your opponent to move themselves. Moving yourself is the simplest and easiest of the three. Moving your opponent is simple but not easy. The third is easy, but not simple. This usually requires an attempted sweep or submission that forces your opponent's hand, but often undisciplined players will simply do the opposite of whatever you seem to want. Don't discount this, push and see if they push back.
By understanding the construct that is the jiu jitsu guard, and what it needs to do gives players flexibility. In the Keenan Cornelius 'de la worm guard'/Jeff Glover 'donkey guard' sense; to be able to take grips and controls that they are comfortable with to effect a viable guard but also to troubleshoot a guard that isn't viable. Either to dissect a roll and evaluate what about their guard isn't effective in retrospect, but also in the moment to be able to repair and reestablish a compromised guard before the pass. This is the difference between high level guard players and normal humans: the ability to rebuild the dam before it leaks, and the best way to get there is honest evaluation of one's performance. So train, evaluate, make changes and repeat.

Feel free to comment with any disagreements, or omissions.

Mahalo