Like many long-term endeavors, to succeed in investing you must “know thyself”
That is to say, the better you know yourself, your own temperament and risk tolerance, the better chance you have of succeeding,
A great way to determine your best long-term investment strategy is to take a short asset allocation/risk tolerance test at the following link: https://personal.vanguard.com/us/FundsInvQuestionnaire
This test will determine your optimal mix of stocks, bonds, and cash reserves.
If practical, consider a disciplined dollar cost average approach which involves contributing a fixed dollar amount at certain intervals such as investing once a month in a 401(k). This strategy will have the effect of buying more shares when the market is down and fewer shares when the market is more expensive which will keep your costs low. In addition, you will have the great advantage of keeping emotion out of your investing.
Wishing you all the best of good buys,