The lime juice should prevent the avocado from discolouring but it is still better to eat it sooner rather than later if possible.
Ingredients
2 ripe avocados
1 large or 2 small ripe tomatoes
Handful of coriander
Juice of one lime
1 red onion (finely chopped)
1 chilli
Method
Put the flesh of the avocados into a bowl and mash it up a little with the back of a fork. Finely slice the onion, chilli and tomatoes before adding to the avocado. Squeeze in the juice of a lime and stir in some chopped coriander leaves.
The everyday vegetarian UK 
I actually made almost exactly this earlier today! Forgot the coriander though… Thanks for sharing!
No problem!