I usually find that, sooner or later, I simply have to hammer a phrase into my head until I remember it. In the case of new languages, it's usually sooner.
Yes, there are useful methods such as mnemonics to remember certain phrases, but don't be afraid to simply repeat, repeat then repeat a phrase until you use it out of habit, without thinking about it or analysing it. It may be a new "lynchpin" for a whole new set of phrases. :)