My ETF profile, my only investment outside my retirement accounts, are based on the ideal portfolio pitched on the MoneyChimp. They go over how to create a complete stock account, using either index mutual funds or ETFs, following some sound financial principals and the Fama and French test portfolios. They cover the complete foundation of the portfolio they suggest starting here. If you're pressed for time, here's the quick version:
1) The largest portion of your portfolio should be invested in the Total US market.
2) Small value stocks perform best historically, and holding a portion of your investments in a small cap fund will improve your performance without adding much volatility.
3) Adding 'some portion' of foreign stock will help to diversify your holdings.
4) Total Market funds are overweighed in large cap stocks (the Pareto problem) and total foreign market funds are overweighed in the stock from developed countries (Pareto, again). Thus, it is recommended that you add an extended market fund and an emerging market fund in order to get more representative market coverage.
The final profile they recommend is found here, and looks like this:
50% Total US Market
25% Extended Market
15% Foreign Market (including an undefined amount of an Emerging Market fund)
10% Small Cap Value
This was the starting point of my own ETF portfolio, but I made a few modifications to it, based on my thoughts and other research. First, I doubled the amount of foreign market exposure; I've heard a wide range for the proportion of foreign stock to hold, from 15% to 50%, and I think that increasing my exposure to foreign markets will help with my diversification. Second, I bumped up my extended market investment to 30%; I don't think the MoneyChimp portfolio went far enough in compensating for the Pareto problem (plus, small and mid cap stocks tend to outperform large cap stocks). By taking this amount out of the Total US stock fund and leaving the small cap value allocation, we get a final allocation as follows:
30% Total US Market (TMW)
30% Extended Market (VXF)
10% Small Cap Value (VBR)
20% Total Foreign (VEU)
10% Emerging Markets (VWO)
I feel this will serve me well as I continue to work and invest for the future.