Q:

How can I keep guacamole from turning brown?

Even though a I put plastic wrap over a bowl of guacamole, it seems it's never tight enough, and the top layer of the guacamole turns brown, which sometimes changes the taste.
Is there any way I can keep my guacamole from oxidising in the fridge?

A:

As has been mentioned here (including a few times by me), this question has been discussed on Cooking.SE here. 
Since I wrote that answer, I have done further experimentation and have found that adding Vitamin C in the concentration of 100mg to 50 grams of avocado will keep the avocado green for days, even exposed to air in a normal sealed tupperware and even in the face of lemon, lime, or vinegar; all of which speed the browning of avocados. The efficacy data of Vitamin C and propanethiol S-oxide gas, caused by diced onions, will be added to that answer.
EDIT I actually ended up writing a new answer after quite of back and forth about how best to handle it. So, it's a bit later than I had hoped, but it's here

A:

I'm sorry, but I have to contradict Cathie's answer:

Which acid works best to keep avocados from browning?
Answer: None (of the acids tested)
It's not that acid doesn't do much to help.
ALL OF THE ACIDS TESTED CAUSED AVOCADOS TO BECOME MORE BROWN AND TO
  BECOME BROWN FASTER THAN NO TREATMENT AT ALL

Source: An answer by  Jolenealaska at cooking.SE.
The pit acts as a oxygen-barrier, not as a preservative. You could use wrapping film instead.

