One guard to pull them all, and in the darkness sweep them

There are a myriad of different guards available. Not even world class athletes whip out all of them at once.

• Pick an "A-game" guard, and a couple backups. 
• Regardless of what your "A-game" guard is, you need to know how to keep the passer at bay at the very least whether they kneel or stand
• If your "A-game" guard cannot be readily pulled right off the bat, you need to know some "pull-friendly" guards from which you can enter your A-game guard. 
• Aspiring deep half or X-guard specialists take note.
• Study how to get to your favourite guard from all the nine bajillion different guards
• If you specialize in a standing guard, but you are instead dealing with a kneeling passer, you will need to adapt your guard. For example, De la Riva can be adapted against combat-basing opponents. Otherwise, deliberately encourage them to stand up in some way, or physically make them stand up for that matter.
• On the other hand, if you specialize in a kneeling guard, but your opponent is standing, try to find a way snap them back down to Earth, whether psychologically (with baiting) or physically (with a technique). 
• Technique example: from detached "butterfly" against a standing opponent, arm drag him back to reality, oh there goes gravity!
• It is one thing to adjust to an opponent depending on their passing tactics (e.g. the opponent stands up in your spider guard, so you wrap a lasso), but it is quite another to use a guard that your opponent wants you to use. In particular, if you are forced into half-guard, where you are just about on your back barely holding onto a leg with the triangle lock, you are even more fucked. Granted, you can bridge-and-roll your opponent much like you would in bottom mount, use reverse half (counter the back step pass) or lockdown (lift the passer if you find yourself in flattened half guard), etc. but still.
• If you get your favourite guard fully locked on, launch attacks. Otherwise, focus on trying not to let the opponent pass, or transitioning to and establishing your favourite guard.