When I first started I looked for high yield / high risk mutual funds that the company offered and participated heavily in the ESPP plan of the company. I picked similar things for my ROTH contributions. This means that I was impacted by the stock market collapse in 2008. It also means that I recovered my losses and was back where I was pre collapse in ~ nine months because we didn't panic and are in it for the long term. I'm 36 and will likely start transitioning some of the investments into 'safer' things when I am around 45 or so. The accounts are checked 2x per year and adjustments are sometimes made but usually we just run the calculator that estimates how much we will have when we are ~55.
The company I started this with offered seminars for retirement investing and there is a chart, sort of a guideline, that details how much risk your investment portfolio should have in it based on how long you have to retirement. If anyones interested I'll see if I can dig it up.
Not that I plan on retiring, even when I am retired but if I was 70 and my portfolio took the hit it did in 2008 I might not have the time to wait for the markets to correct. Note: I know very little about finance, investing or otherwise. My wife is the financial guru in the family and I have learned much from watching her lead. Please take this as what worked for me and realize that ymmv. Also, it helps to read up on the basics of investing for retirement but i'll be honest when I say that it has been several years since I read anything.
[edit] - I might have misread the question. Sorry. This is what I did when I was starting at 24 and would do again if I was just starting out. I doubt that I would pay someone else to do it for me since the fees that are charged when one sells/buys end up taking a significant chunk and any service is going to ad their fees on top of those.