Do you want to learn new vocabulary in a very efficient way? Use flashcards

Keep it short and simple. Do not overuse complicated vocabulary if you can say the same thing in a fewer and simpler words. Also: do not build too long sentences. Firstly, they might be too difficult for your partners and secondly, you yourself can get lost in them and forget what you wanted to say. This would be quite embarrassing, so try to avoid such situations at all costs.