If you've already mashed the avocado, you can keep it from browning by adding a little bit of ascorbic acid (vitamin C). Here[0] is a series of tests someone else did to prove that this works. I've personally had guacamole last several days in the fridge with no noticeable browning, but then again, guacamole doesn't last long in my house once it's been made. :)
Lime juice is a staple in my recipe. Have substituted with lemon juice when I'm surprised to not have lime available. It helps with the browning but not as much with the flavor
The link in my comment suggests that citrus juices actually _speed up_ browning, and I've seen other sources that corroborate this. Do it for flavor, certainly, but I wouldn't do it for preservation reasons!
[0]: https://cooking.stackexchange.com/a/53406