Is it better to say "Don't forget" or "Remember" in written encouragement? This is more of a subtlety and goes beyond general English Language &amp; Usage and is more about the cognitive process surrounding the use of language.

It seems like I heard once that, while it is common to say, using a phrase like "Don't forget" or "don't miss" is essentially a cognitive double negative. Consciously we understand that "Don't forget" means to "remember" but unconsciously our minds drop the "don't" and file it as "forget," thus conveying the opposite message.

The argument goes that if I tell you "don't think of a pink elephant" then you need to first think of the pink elephant before you can cognitively not think of it. Later if you recall back to the conversation you would remember it was about pink elephants and not thinking of them.
I'd argue that Churchill's "Never,  never, NEVER give up" didn't reputedly have this effect. 

With 'Don't forget' the 'don't' may well outweigh the infinitive verb in its cognitive effect, especially if stressed. The language here is of stimulation, incentivising, or command, and I'd say the more likely undesired reaction is the dislike of the perceived patronising (or, certainly, the imperious) approach. 

With 'Don't think of a pink elephant' there is little likelihood that one feels patronised or browbeaten.