This is a simple one-step Instructable which I learned from Nidge at Brixton Cycles in London (a genuine worker co-operative).
Always wrap plastic wrap (cling film) around your vulcanising fluid before putting it away (after you have put the lid on of course!). This makes a nice airtight environment to preserve it.
I've just fixed flats in 8 tubes with some solution which has been on my shelf for months. I was surprised that the solution seemed brand new - not partially dried up or tacky.