Keep your investment fees (otherwise known as the expense ratio) low–this means on everything you have invested: your IRA/401k/403b/457b/taxable investments. This interactive infographic from PBS’s Frontline explains what fees are and why you should keep them as low as you can. If that doesn’t light a fire under your butt and get you looking at what fees/expense ratio you are paying, I don’t know what will.
So, what’s a “low” fee? If you are looking at your employer’s list of 401k investment options, you should be able to compare their expense ratios as well their rates of return. But for a comparison, Mr. Money Mustache likes the Vanguard VTSMX index which has an expense ratio of 0.17%. I have a Fidelity IRA in FUSEX, which has an expense ratio of 0.10%. And my 457b with my employer is in VIIIX, which has an expense ratio of 0.025%.
You might also notice the examples I’ve given are all index funds. That’s because actively managed funds generally have higher fees than a straightforward index fund which simply tracks the S&P 500. You can find a million other articles about index funds, or you can read one book: John Bogle’s Little Book of Common Sense Investing (preferably free from your local library). I read most of it in a weekend, and then immediately switched a bunch of our mutual funds to index funds. Fun stuff.