I found a list of 10 great tips in an unexpected place – Wikihow. Wikihow is a collaborative site with how-to manuals on almost every subject, including writing thrash riffs apparently. The tips are simple but really very good and most of them can be applied to writing music in general.
Here’s the list in summarized form:
- Know your thrash metal – with few exceptions, you won’t be able to write any style of music that you aren’t intrinsically familiar with.
- Learn scales and chord progressions
- Learn alternate picking and palm muting.
- Don’t give up. Writing songs takes time, especially at first. Be prepared to spend time to do it properly
- Learn to play riffs from other bands and make your own variations (for practice and knowledge, don’t keep anything that’s too much like a rip-off). This is a bit like point 1 but taken to the next level.
- Work with your riff and then decide if it makes the cut. Starting out with the riff in simple form is a good idea, then embellish, change and refine. Be prepared to just discard riffs if they don’t turn out good or are too similar to existing songs.
- Try your riffs on other musicians, like members of your band (if you’re in one).
- If you can’t play your riffs fast enough, use double picking (see below)
- Write riffs in different speeds, thrash isn’t just the fast parts
- Be open – try not to get stuck in too many preconceptions about the genre.
Here’s the whole wikihow article.
Double picking is new to me – it means playing each note of a pattern twice, a sort of specialized variant of tremolo picking. Your picking hand is usually quick enough, it’s often the the sync between the hands that fails. Double picking might seem a bit like cheating but can actually sound pretty good and is a good way of practicing synchronization between your hands. Here’s an example of double picking: