When throwing a party, the cost of providing alcohol for all the attendees can become quite steep. This is especially true for bigger parties, or if more people show up than you were counting on. With the exception of potlucks, it is generally assumed that it is the host's responsibility to provide some kind of snacks or refreshments, but it is perfectly acceptable to ask your guests to help by bringing their own drinks. Ideally, your invitation should make it clear that this is expected but without being too explicit.
Write the invitation normally, explaining the time, date, and location of the party, as well as any particular reason for throwing it or special requirements such as costumes.
Write "BYOB" at the end of the invitation. Make it is large enough that it will be noticed. Don't worry about spelling it out -- the acronym is nearly universally understood to stand for "bring your own booze," "bring your own beer," or "bring your own bottle."
Add an additional request, possibly as a postscript, for people to contribute a few dollars if they are unable to bring their own drinks. Even if you ask people to bring their own bottle, some will inevitably forget or be in too much of a hurry. By asking for a monetary contribution, you can either offset the costs of whatever drinks you do purchase or provide the funds for a last-minute beer run.
- Depending on who you're inviting, you might want to use words such as "drinks" or "beverages" instead of "booze" or "alcohol." The meaning will be understood without seeming crude.