First of all, give em a hard deadline if you need to. If it's chill then no rush, but if you need it by a certain time make sure they know. Also make sure they know that you maintain the right to not use whatever they send you if it doesn't fit your vision. Pay them after the song is completed and ready to upload. If we're talking you doing features and not just receiving them, only work for free if the artist is either bigger than you or the song is really good. Otherwise, make some money outta the deal.