Whatever your drug of choice, remember: drugs and alcohol can affect your ability to make decisions (such as identifying danger signs and plotting escape routes). Manage how much you consume.

Always get your own drink. Watch it being poured.
Don't leave it unattended.
Don't drink or taste anybody else's drink.
Don't accept drinks from anyone else.
Dispose of your drink if you think it tastes odd.
(if you just leave it, someone else may drink it).
Drugs used to spike drinks can be colorless and tasteless so you might not realize that anything has been added.
Be aware of the behavior of your friends. Is their behavior out of character? Do they seem too "out of it" for what they've had? See the section on Drink Spiking to find out what to do.
Watch out for your friends. Are they binge drinking or taking drugs? Are they making safe decisions? Are they OK?
If your friend needs to sober up or straighten out, check to see that they are OK. Are they alone? Who is with them? Are they safe?
Encourage people who are drunk to drink water or eat something. Keep an eye on them, if you can.
Do you really trust whom you are going home with? Are you sure they won't make you do anything you don't want to do? Will they take "NO" for an answer?
If you are going home with someone different, let someone know. Introduce them to your friends.
If You're Throwing the Party

Know whom you have invited to the party and plan what you will do if there are gatecrashers.
Provide food and non-alcoholic drink spacers, e.g. water, soft drinks, juice.
Keep a look out for your guests. Make sure they're safe and having a good time.
Offer to organize taxis for your guests. Take a note of the taxi company you have called.
Create a safe atmosphere. If you know that someone is harassing or intimidating someone else, then do something about it:
Let them know you are watching;
Ask the person to stop;
Ask them to leave;
Find the victimized person a safe place;
Don't be afraid to call the police.
Stay safe yourself and ask for help if you need it.