Always think about how you’re going to grab the next crate. There’s no point idling around because your score isn’t going to increase unless you keep moving.
Be careful how many enemies you allow past you. One or two angered enemies can be dealt with easily enough, but too many coming from both sides will be big trouble.
If the amount of enemies is getting a bit out of hand and your current weapon is good, it’s not a bad idea to clear the screen of monsters before grabbing the next crate.
Learn how each enemy moves through the arena. They’re very predictable for the most part, and knowing where the enemies dropping from the top of the screen are going to go is helpful.
When you unlock a new weapon, it’s not a bad idea to set aside crate collecting for a little while and spend some time learning how it works. You won’t have time to think about things in a serious run, so knowing what each one does can save your life.