When meeting up with friends, plan ahead and fix the venue and time in advance. Make sure everyone sticks to the arrangement.
Memorise the cellphone numbers of the people you are meeting. In case anything happens, you can still call them from a borrowed phone or a payphone.
Get a phone card!
Always prepare your friends for the worst. For example, if you left the house 15 minutes late, tell them you might reach the venue half an hour after the original meeting time. This ensures that your friends will still be there if you arrive 20 minutes late.
Finally, it is good to keep your parents informed of your whereabouts. When you are setting off to meet your friends, inform your parents. This way, if you do not appear at the arranged time, your friends can call your house and ask your parents what time you left.