I've been using Vaseline (Petroleum Jelly) on my maglite for years and have never dissolved an O-ring to the point of disintegrating or breaking apart. It's cheap and has worked, even Maglite recommends it too
http://www.maglite.com/faq_details.asp?faqProd=D
I have heard people using chap-stick to lubricate the o-ring as well.
If you are able to find some dielectric grease, they are economical as well.
I use Nyogel, a small tube I have has lasted for 3 years, and is still 90% full. When you factor in how long it last, it works out very economical