I've had very good luck with my solo 401k. I max that out after I max out my 401k. I let my accountant tell me how much I can contribute to last year's solo 401k after she does my taxes for the year. You can contribute to last year's account until April 15 of the current year. (But unfortunately, you have to open the account by Dec. 31.
After you do that? I thought that after you max out your 401k you can't contribute anything to your solo 401k (as an employee). ??
That's a fair question, and information is pretty hard to find on the web.
My "regular" 401k is through a contract job for which I get paid as a W-2 employee. I contribute the annual max to that and get the employer match.
My solo 401k is through my own business. I can add up to 20% of my business profit as "profit sharing" to my solo 401k.
The combined maximum for both accounts is $53,000 for 2015 (or $59,000 for those 50 and over).
ETA: Don't take my word for it, though. Talk to an account for possibly more accurate information.