Is it better to say "Don't forget" or "Remember" in written encouragement?
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.