Quoting the wisdom of others in your speech can help to make a point. However, you must always remember that the audience has come to hear you, not someone else's words. When you quote someone, always attribute the quote to its original source. Make sure too that it is someone your audience will have heard of. It makes no difference to them what John B Snodgrass said if they have no idea who he is. You may help them by qualifying the name, such as "The great philosopher Plato once said...", but you shouldn't really have to.
Quotes can be over-used too, so avoid using familiar ones like "Ask not what your country can do for you..." (especially at the moment, with comparisons between Obama and Kennedy).
Quotes can be direct (where you use the precise words) or indirect, where you paraphrase. It doesn't matter as long as it makes sense, and is in context with the rest your speech.
Don't say "and I quote.." Don't ever use that awful gesture where you draw quotation marks in the air with your fingers. And never, ever, quote yourself. That way lies madness.