There aren't too many rules here and but are some guidelines you may want to follow if you don't want to embarrass yourself and/or insult anyone. I will also give a few line you can include in your own speech.

It's a Wedding - It's not all about the stag and speech there is more to your role, keep that in mind.
Be Thoughtful - why not buy a nice bottle of champagne or wine for the honeymoon suite for after the wedding. Offer to pay for your own suit or the hire?
Take the Stress - Unfortunately, you are there for the entire day from prep till the tidying up. like tip 1 it's not just about the speech and stag. You should be the go to guy unless it's a real emergency the groom should know of anything that has turned up late.
Be Prepared - Show that you care about the wedding as much as the groom, get everyone's number from the bridesmaids to the vendors. keep the bride and groom hydrated and make sure the vendors have all paid and been fed and have what they need to do what they were hired to do.
Keep your speech short and sweet and avoid awkward stories that are funny ' by the bar' stories, to a minimum. the aunts and uncles and grandparents don't want to hear about the newlyweds' sex life. Also its generally a good idea not to bring up any ex's during your speech.