Okay, for an ETF, especially a broad index or sector index, I would figure out your amount and set a time for normal buying. For our brokerage E-Trade account (which consists of 8 ETFs), we deposit the same amount every other week. I have been buying fully every 2 weeks since all the ETFs are way below all-time highs. Keep that in mind, if you start an ETF position now, you are already getting it lower that before. Over time, indexes go up.
Now, to be honest, I do adjust the buy timing by a day or so in case the market is doing something crazy, but in general, I stick to the plan.
As for stocks, that really depends on the specific stock. I'm not a stock guy. I play the market mostly with ETFs, funds, and leveraged ETFs.