Fantasy larps are typically set in the medieval-ish period, and feature recreating the atmosphere to some degree. And while few larps have requirements as strict as living history or reenactment groups (both for cost reasons and the fact that fantasy styling is not always medieval).
However, looking back at how they did it in Middle Ages can teach us a lot. Digging deeper into medieval lifestyle makes for a much more immersive larp experience, since it's more different than what you do in your regular life.
I'll talk about mailny two things here - clothing and diet. The reason why I chose to talk about them was that they're most visible difference - to those around us and to ourselves. And, they're easy to simulate - actually most advices here will be cheaper and healthier than what people usually do on larp.
They might seem drastic, but they're actually easy to get in. Then again, most people who read this won't follow up on the advice because it's probably out of your comfort sphere... but please consider them.
Diet is where you can save a lot. Grand feasts were usually the privilege of higher nobility. Poorer classes had a much different diet. Gruel and pottage were staple food - in first, you cook some grain in water (like oatmeal, grains need to be at least cracked), and in others some vegetables. Cook for a long time. You may add a bit of salt. It might not be the tastiest thing in the world, but it's actually healthier and cheaper than what most people eat today.
Also, people were often fasting, in various ways. Some recent research says that fasting (done in moderation, of course) can be healthy. Either way, it's different than today's lifestyle (not to mention free).
Clothes were done with specific patterns and of specific materials. Patterns were usually not cheap and simple - as most larp clothes is - but rather complex. Main guideline was to achieve savings on material used, and for a good reason. More material meant more work, because you didn't just buy the material for a cheap price: you usually had to shear a sheep, spin the wool into a thread, weave a fabric you desire, and then tailor it down and sew. All by hand of course.
You probably won't have the same opportunity to repeat the entire process from the beginning. But, if you use authentic designs and materials (such as linen, wool, fur etc) you'll find them to be quite functional. And besides, synthetic materials just don't have the same feel. By the way, this advice is the only one around here which will probably cost you more money, as natural materials such as wool stuff are quite expensive.
Good footwear for larp is probably the hardest thing to obtain, especially if you're a guy - women typically have a selection of passable boots and sandals, but men have a hard time finding medieval-ish footwear. Luckilly, you have a cheaper alternative - go barefoot! (unless you're playing a noble, again) It takes some time getting used to if you don't usually walk barefoot, but if you practice a few weeks you should be fine in nearly all conditions. And it's healthy. Just be careful not to get stepped on.
There. If you follow any of those advices, they'll probably make your larp experience deeper - by offering new sensations during larp - while they might improve your health and your wallet situation (except for the one with cloth types). They'll also improve others' experience as you'll put other players into more authentic situations.