The people who traditionally give toasts are, for the most part, already involved in the wedding. The one "mandatory" toast to have, if you have no others, is the best man's toast to the bride. Many couples stop right here and let the guests enjoy their meal or dancing with no further ado.
A common modern practice is to have only two toasts: the best man's toast to the couple, and the maid of honor's toast to the couple.
It's also not unusual for the groom or the bride to give a toast in response to the toasts by their honor attendants.
If you are longing for an elaborate and somewhat arbitrary sequence of toasts, protocol expert Letitia Baldridge provides it. Her order of toasts is:
This elaborate scheme brings with it three significant downsides.
A more enjoyable sequence of toasts can be arranged by following the best man's toast with a small number of toasters who are chosen because they're both close friends and skilled at expressing themselves well. Or, if your circle of friends includes many people who might want to speak, and who might do it eloquently, the best man can ask who else would like to give a toast, and see who volunteers. It's perfectly proper to have toasts done by people who want to do them, regardless of whether they're on the "official" list. People who do not express themselves well are better thanked when you or your new spouse stand up and give a brief speech thanking those who are important to you.
An effective toast is brief and not too silly. The toast-giver should be prepared to tell one to three short, non-embarrassing stories that illustrate important characteristics of the bride and groom that will make their marriage together happy. If the toast-giver does not wish to tell stories, he or she can also stand up, say something like "To John and Jean, may they always be as happy as they are today," and sit down. Thanks can be incorporated into a toast, but it should not resemble an Oscar acceptance speech.
As far as asking people now to give toasts at a wedding in August, your enthusiasm is both understandable and commendable, but it's likely to backfire on you. Even your most devoted friends won't remember the request and if they do remember it, they won't act on it until a few weeks before the wedding anyway.
Concentrate your energy on planning the ceremony! Today, many officiants expect the happy couple to have a lot of input into the form and wording of the ceremony. Planning an effective ceremony turns into a remarkable amount of work, and your decisions will affect people who will need considerable advance notice to do their parts well.