Until you've pulled together a significant nest egg, the single most important thing is getting into the saving habit.
Don't wait. Lookin' at you, thirtysomethings. Retirement expert Wade Pfau's research makes it clear that if you wait too long to start, you may have to save heroic amounts to catch up -- or hope you have plenty of bull market years to help you along. Your other option: Delay retirement.
Some risk helps. You can save a bit less if you can handle some risk. But there's a cost to that as well.
Look beyond your 401(k) if you can. The highest-paid workers actually contribute a smaller share of income to 401(k)s because they run into caps on contributions. So if you are a high earner, save more than your 401(k) max.
If you also bump into income limits on IRAs, there's a backdoor: Contribute to a nondeductible IRA and then convert to a Roth IRA. (Caveat: If you hold other traditional IRAs, tax treatment can be tricky.)