I've been playing poker seriously for about 3 months now and have been focusing a lot on reviewing my hand histories. I play online and use Holdem Manager. I was curious how others study their hand histories.

My strategy is to take note of significant hands, whether I felt I didn't know what to do, I questioned my play, I suffered particularly bad variance, or I suffered particularly good variance, etc.. I would say per 1000 hands I spend 1-2 hours reviewing.

While reviewing here are some of the things I do...
1. Write down mistakes I make
2. Post questionable hands on 2p2
3. Separate losses I have from my own mistakes vs. losses I have from bad variance
4. I note when I start to tilt, what caused it, how it effected me, etc.
5. Most importantly, I spend time thinking about possible ranges the V's could have had that I didn't have time to while playing. I do this in hopes of getting faster so I could make better decisions on the spot. This is currently a big leak for me, often I cant think fast enough with the 30s online limit.

What are some other strategies to use while studying hand histories?

Sounds like you're doing all the right things and have a good system in place.

I think the key with hand histories is to also have discussion and feedback from someone else. Otherwise it's just our own thoughts. Posting hands on here is a great way to get feedback but so is interaction with a coach. Unfortunately, they can be a bit pricey so probably best to only do it once a month or two months.

I tried a hand history review session with Texas Holdem Questions and whilst I found it very useful, it isn't something I can afford to do regularly.

All good stuff.

I would suggest finding ways to incorporate more quality feedback - ie solver, training videos, coaching, chat with strong players, books...