One bad habit, I call my passive index funds my "core", but at this point they have become 1/4th of my portfolio. I wouldn't recommend others follow my 50/50 split, since the goal is to avoid losing money. There should be a diversification benefit, but it can take 10-20 years to play out. Some decades the U.S. beats international, other decades international beats U.S. stock performance. A Vanguard white paper I read suggested 20% international has the most certain diversification benefit, so I usually recommend that to others. But here's what I hold:
1/6 MTUM (I like it better than Vanguard's U.S. Momentum ETF, VFMO)
1/3 ITOT (VTI is the same)
1/3 IXUS (equal to VXUS)
1/6 IEMG (same as VWO)
Vanguard doesn't disclose how they calculate momentum, so VFMO is opaque. There's no index, just Vanguard saying how it works. I prefer an external index, like Vanguard uses for it's passive index funds. So MTUM tracks an MSCI momentum index, and the methods are well designed enough that I was impressed... the 0.15% expense ratio probably sealed the deal. (VFMO charges 0.13%, and holds smaller companies)
Note if you make a 9-box portfolio (small/mid/large) x (value/blend/growth), you create fixed percentages at one point in time. The total stock market may drift, shifting more weight into large cap / growth. The stocks with the heaviest weight in the U.S. stock market are Apple, Microsoft, Amazon, Facebook, Google. All tech stocks, all in the "large/growth" category. In 2020 I think they collectively gained +50%, significantly pushing up the performance of the entire stock market. These are huge stocks, and if you rebalance by selling large/growth to buy large/value, it's a form of underweighting them. So that's the risk of picking a 9-box approach, as I understand it.