my mom has always tryed to teach me proper etiquette for many things (as you see it has not worked very well since i cannot even spell the word) but this is what is said about weddings:::

witness weddings---you give what you can and people should understand that. if you don't have money at that time, you still write them a card congratulating them and promise yourself to catch up with them within the next 6 months. sometimes by then a couple has run outta money anyway and will need that help. you give because you love and want them to start a wonderful life together. but you should plan in advance because you obviously know months ahead that you will be attending their wedding.