1. Have some ideas ready to use (ready made sentences, introductions, structures, turn taking expressions, agreement, idioms, ...)
2. Prepare and use a certain amount of high-level vocabulary (strong adjectives, adverbs, specific or topic-related vocab, ...)
3. Improve your pronunciation (be careful not to repeat basic pronunciation errors and practice using phonetics and different sources)
4. Linking words (there are never enough!)
5. Use grammar structures in accordance to the level (passive voice, reported speech, conditionals, modal verbs, infinitives and gerunds, perfect tenses, relative clauses, comparatives and superlatives, ...)
And finally a video explaining in more detail this checklist: