There are others here that know far more about this than I do but based on what I have learned from previous discussions and research on the subject I believe that your best bet is charcoal filtering and/or the evaporation/condensation method. Evaporation/condesation methods such as solar stills are often difficult or impractical to set up in a manner that would yield enough water to justify the effort involved. Charcoal filtering is easier to improvise than most evaporation/condesation rigs but still easier said than done, so practicing before youre in a situation would be extremely beneficial.

It would seem to me that in many situations clean water is easier to find then most may suspect. Dew is an excellent source of clean water as long as you're not pulling it off of poisonous plants. Rain water is also a great source of clean water provided you're not in an area known to have excessive air pollution. A variety of non-poisonous plants are also a source of clean water.