Wow, nothing like throwing yourself in at the deep end, Lady Luck! 30 - 40 people? I panic at more than four LOL.
Some good ideas from folks above here - I definately agree that it will be good to have a buffet, not a sit-down. Keep the place moving & lively.
A few tips to take or leave ...
Labelling
Put tags with the dishes/trays or print menus and post them on walls or in a frame on the serving table - that way you can specify what everything is. Then people can sort out their own dietary requirements and this avoids "is that chicken or tofu?" worries for your guests. Colour code serving trays, even. Red for meat, blue for vego type thing.
Lots of
variety - have some vego , some meaty stuff. Instead of serving with sauces/dressings already on (eg pasta, salads, desserts) have them next to it so people can pick the one they like.
Try to miminise cutlery & messy stuff - pre cut lasagnes, roasts etc, and serve soft /small foods that don't need too much chopping up - it's hard to cut anything with a plastic knife while standing up.
You're going to need mega table/bench space - hire trestles if you can & stick everybody out in the backyard (pray no rain LOL!) This also miminises the spill&stain agony factor.
Lastly, if you can,
keep your kitchen clear. Serve the food somewhere else, that will help your sanity on the night. As hostess, you'll need somewhere to work, and also somewhere to catch a minute's quiet occasionally. You don't want to be dealing with guests trying to find drinking glasses while you're juggling birthday cakes & trays hot out of the oven. Beg, borrow or steal a large urn from somewhere so people can make a cuppa without invading the kitchen.
Good luck & have a brilliant night!
Cheers ~Ultra