What I do is get a very soft (totally clean - straight out of the box!) tissue and very slightly wet it (just a few drops of water). I gently wipe the screen with that. No chemicals and don't do it dry, whatever you do! Alternatively, use a very soft "fluff free" cotton sheet (e.g. I often use a corner of a clean pillow case in a hotel room when traveling with work) and again wet it very slightly
with a few drops of water. Best not to use any tissues that produce a lot of "dust/fine fibers" or which have creams/moisturisers etc. impregnated in them (to "make" them feel softer).
I've used this technique on both of my AV420's for 2 years and my 604 wifi (for 2 months) as well as on other optical devices, e.g camcorder lenses and touch screens, laptop/PC screens, SLR camera lenses etc. over the years and have never scratched any of them. None have ever had (or so far, touch wood, needed) screen protectors but then I'm carefull with all my kit - but it all gets heavy use as I'm a frequent long haul flyer with my job, constantly traveling all over - off to Brazil yet again today for a week!
The key is just enough moisture to be a lubricant, not pressing too hard and continually presenting a clean/very slightly damp area of the cleaning cloth to the screen/lense.
If you can see a bit of dirt be very careful removing it for fear of it scratching the screen at it moves around (especially if your press too hard). One idea is to dab it a few times with a slightly wetter patch of tissue and let it absorb some of the moisture before then trying to remove it. Never allow any "free water" around the edges of the screen (
I guess a legal disclaimer should now follow!!! Just my suggestion/experience on this and good luck. Oh, and the people who pocket their 604 wifi's with their small change won't need to read this - their screens will be scratched horribly already! Anyway, hope this helps.