Write only 1-2 actions per post because it makes it easier for the partner to react to what is going on. It also reduces the chances that you will force a player to go along with something they don't want to. (Especially if many actions get buried together as it can be tough to sort out what is going on.)
Keep actions fairly brief, though described well so your PvP partner knows what you're trying to do but not is overwhelmed by descriptions. (EX- If your character swings a sword… include the motion of the swing and direction but don't bother going on about the blade's history or how shiny it is or it's last victim. The latter stuff is filler and can make it tougher for your partner to reply to.)
Don't go too far into future actions. This means avoid saying a lot of things such as: “If the opponent went right then Dante would go left.” because it can be misinterpreted very easily and cause people to think you're either controlling their character or can cause confusion as to what your character actually has done. So at best use only 1 or 2 of these actions per post. (For best results, only use these at the end of your post so the other player knows what your intention is, or if you're guarding an attack so they know what sort of counter attack your character would make.)
Write actions only for your character. This means don't decide if an attack hits the other player or if they dodge/defend against it, let them make that call for themselves. (Though if a player dodges everything or makes you take damage like this… Seek a staff member.) Also don't force a character to move in a certain direction or to a new location (unless they want to and it's been agreed on first.)
You can make attacks miss, projectile type attacks especially, and those can cause damage to the surrounding environment… Use that to your advantage. Describe what's happening to the environment as your attacks land/miss.