Marcus’s Tripod Site – the original Blades website, with all the original silliness.
LARP Hacks – Tumblr where people can ask questions on any larp subject and get useful answers.
Ladies Who LARP – A celebration of Ladies Who LARP. Includes such things as how to make make-up stay put for a day in the field.
Gamesmastery 101 – Technically about tabletop but a lot of the articles can be extrapolated to larp.
access:LARP – A relatively new group working on accessibility issues (physical and mental) in LARP and how to resolve them, both for organisers and players.
larprambles – Thoughts on aspects of LARP from a relatively new LARPer, very much worth a read.
General Other
RPGNet – General website for all aspects of roleplaying, interesting articles and sometimes useful advice.
Tales from the Table – User-submitted stories of when things went unexpectedly and often hilariously right or wrong in tabletop games.
GMB – guides to sorting out physical fitness between games, especially useful for heavy warriors!
Antimony & Lace – A number of useful simple sewing projects like long vests and pirate shirts.
Alleycat Scratch – Meant for making LotR cosplay kit, a lot of useful projects and tips for everything from clay and metal to leather and fabric.
Choly Knight – Technically about making plushies but the Tutorials section is useful for a budding costume maker.
Clothing Patterns 101 – How to do full tailoring and pattern design from scratch for your particular body shape.
Historical Underwear: Corsets and Crinolines – This one is more abstract, but it’s a useful resource for things to think about under your kit (although from a safety note crinolines can get interesting in woodland!).
Cosplay Tutorial – Tutorials for any aspect of cosplay, from costume to prosthetics to makeup.