To create variation, know enough theory of what notes you're playing, how they relate to each other as scale degrees, as well as the duration of each note and where the strong beats are. You won't be able to create variation if you don't know what you're varying. Learn to recognise keys by their notes.
A quick way to create variation: take one of your riffs, divide it into logical groups (such as 1:[note chug chug] 2:[note note chug chug] 3:[note note note chug] 4:[note note note] 5:[chug chug chug note] [repeat all]) and reorder the sections so you have so you may instead have 3, 4, 2, 5, 1. It may sound like balls but it may give you an idea for something better.
This basically gives each section a slightly different context, which can be a very important part of keeping things interesting. There are more ways to develop than just more notes/harmony and context is one of them.