Others have hit the key points, so I'll add just one other thing: if you're in taxable investing (it makes no difference in tax-advantaged accounts), don't buy the fund right before a dividend -- not only do you just get some of your money back in the form of the dividend, you also pay tax on that amount.
This is the closest to "timing the market" (and it isn't really timing the market in any real sense) I'll ever suggest to anyone - if you're ready to buy more of whatever you're invested in in taxable, check if it's about to issue a dividend. The stock funds typically do this at the end of each quarter (end of March, June, September, and December), bond funds are, I believe, monthly (my bonds in taxable are, as are my bonds in tax-advantaged, but I haven't checked all their major bond funds).
This also makes no difference if you're buying a muni bond fund in taxable.