1. You've already got plans
The first go-to excuse that requires little explanation. This only really works on people who won't ask what you're doing because then you have to lie through your teeth on the spot which can sometimes lead to a Joey-esque Racoon-related excuse.
2. You're shattered from [insert tiring activity]
Whether you exaggerate how late you got in last night or how badly you slept, this is an excuse easy to pull off. Just don't get caught doing something else during this time.
Note: Risk of being called 'boring' and 'never up for anything' if you use this excuse regularly.
3. You're broke
You're a student, unemployed or maybe you just lost all your money at Ascot last week, regardless, whatever your friends have invited you to, it will probably cost you money even if it's only on travel. Most people can't argue with the broke card unless your friends are the generous type which offer to buy your drinks all night or pay for entry. If this happens, you should probably go because they must really like you and are pretty good friends.
4. You forgot about that important family meal
A spin-off of #1 it's pretty difficult to argue with this excuse. Whether it's that Aunt you haven't seen for years or that Uncle who's visiting from Australia, just make sure it's a family member you actually have otherwise you may be caught in a muddle at a later date when the one you've just said no to finds out your parents have no siblings.
5. You've just been called into work
No one can argue with work, see #3, we're all broke.
6. You've contracted an illness
If it's a guy you're telling an excuse to, tell them you've got menstrual cramps. If it's a girl, diarrhoea. Neither are arguable or require any further detail and they will both make the other person feel awkward enough to end your conversation as quickly as possible.
7. You've got to wait at home for a special delivery
Best paired with "I'll come as soon as the delivery comes", which works wonders, leaving it in your hands to decide when the delivery should arrive. Don't want to go out at all? It arrived late, gutted you missed [insert event] - although you and I both known you're not gutted at all.
8. Ignore calls/messages/efforts to contact you from the inviter
Very easy to carry out, requires little or no effort on your part, just the ability to ignore a ringing phone. This then leads to the very easy follow up of "I would've come had I known, annoyed I left my phone off."
9. You're out of the country
This requires a lot of skill to pull off effectively. Check the location settings on your facebook updates and tweets, no instagram selfies of you and your dog at home, and for gods sake don't bump into the person you're avoiding. This excuse should only be used in extreme circumstances when all else fails.
Likelihood of being found out: High.
10. Be honest
You're still in bed, you can't be bothered, you'd rather spend your day watching House and catching up on episodes of Made in Chelsea. Chances are the person asking you to do something won't mind. Hell they might even join you for a duvet day.
Note: If you're the one inviting your friend to do something, the above excuses are not always lies. So don't jump to conclusions. But if they've used 5/10 of these with you the past 5 times you've asked, I'd start questioning your friendship.