How to make less mistakes in math?

I have to take a timed test this summer to determine if I get into AP Stat. I'm decent at math but I make really stupid mistakes. Like one time the number was 236 and I just put 23 in my calculator just dumb stuff like that. So since this test is timed are there any tips of going fast without making too many mistakes? I get how to solve the problem my brain just skips little bitty things -_-