When I set up my 401k, I used the guidelines suggested by The Armchair Millionaire. I bought the book a few years back and have referred to it regularly since then. I have my 401k split up with approximately 1/3 in a U.S. small cap mutual fund, 1/3 in a U.S. large cap mutual fund and 1/3 in an international large cap mutual fund. I would never have considered an international fund on my own. But the book made sense saying that it provides balance to have an international fund in the mix in the event that the U.S. markets are struggling. My international has consistently performed better than my U.S. ones. I may be moving things around to put a little bit more into my international fund but I'll do some additional research first.
I didn't get serious about saving for retirement until I was in my late thirties. Better late than never. I started small so I could at least take advantage of my employer's match. As I got raises, I added to the percentage taken out for my 401k so I didn't notice the change to my take home pay. Thankfully, I've got a while to go until I reach seventy so I'll just keep on saving as much as I can.