Disclaimer: I'm not an accountant, so my answer here is paraphrasing what I've read from others and may not be the best, is not advice, etc. Since the tax code is always changing, the techniques available will change over time. I always recommend having an accountant or financial advisor.
That said, there are several ways to save money on taxes in the situations that exist around early retirement.
1 - One example is the 0% long-term capital gains tax rate accessible to a low income bracket, which one can put themselves in for a given year when they are living off investments and in control of their withdrawals (vs a normal salary that would exclude you from this bracket).
https://www.physicianonfire.com/the-taxman-leaveth-taxes-in-...
2 - A second strategy to consider is "Backdoor Roth" and Roth IRA conversions.
https://www.physicianonfire.com/backdoor/
https://www.gocurrycracker.com/never-pay-taxes-again/
https://www.madfientist.com/how-to-access-retirement-funds-e...
http://www.mymoneydesign.com/personal-finance-2/retirement/b...
3 - One more is maximizing contributions to an HSA as it combines the tax benefits of a Roth IRA (tax-free withdrawals) and a Traditional IRA (tax-deductible contributions) in one account.
https://www.madfientist.com/ultimate-retirement-account/
https://jlcollinsnh.com/2014/08/18/stocks-part-xxv-hsas-more...
4 - Avoid short-term capital gains tax rates. (Compare the percentages in the 2 tables here.)
https://www.investopedia.com/articles/personal-finance/10151...
5 - One last one to keep in mind is tax-loss harvesting and less commonly tax-gain harvesting.
https://www.madfientist.com/tax-loss-harvesting/
https://www.madfientist.com/tax-gain-harvesting/
If you're curious for more FIRE related info, some good subreddits to check out are:
- r/leanfire
- r/financialindependence
- r/personalfinance