@soulplusmind
Here's a few sync tips that have really helped me as a producer.
1. Always end with a sting
2. Instrumentals should have movement, especially if you're working with an artist. Add or take away elements every 4 or 8 bars. Don't just loop something for 3 mins.
3. Depending on the genre, 2-3:30 is usually a good length.
4. Earworm-y simplicity is key. Most times there will be voice over or dialogue so you don't want to overpower that.
Anything I'm forgetting? What's helped you?