Only a few years ago online check-in seemed like a novelty. Airlines would even bribe passengers with extra miles to try it. Now, while no U.S. carrier charges more to check in at the airport, yet, online check-in is becoming increasingly ubiquitous and easier.

In normal human fashion, the easier the process gets, the more casual passengers get about the whole process and the more problems arise. Here are four tips to avoid online check-in problems.

1. Watch the time window for online check-in carefully. It’s usually about 24 hours in advance to two hours before departures. I had an upset administrative assistant call me this week because “United’s site was down and she couldn’t check her boss in for his flight.”

I double-checked, but sure enough, his flight was leaving in an hour, and told her, he’s late. “Oh no,” she said, “He’s at the airport but the lines are long and he wants me to check him in so he can get out of line and go to security.” Well, I had to tell her I couldn’t help. She wasn’t happy but the online check-in was closed. (He did make the flight, barely.)

Similarly another client sent an angry email last month because she couldn’t check in for a flight on a European carrier. In this case, check-in didn’t open until 23 hours in advance. (Which it did say on the site, but she didn’t notice.)

2. When possible, check in earlier rather than later. In the case of Southwest, it means an earlier boarding priority. However, even when timing doesn’t a affect a seat assignment, it means more time to correct potential problems.

In an example from last week, a client was booked to Boston on a United flight, except that United had changed the flight to one operated with a Continental aircraft with a United number. No problem, and the client knew this, but when she checked in on, it said she had no ticket. She emailed me late at night asking if she should just check in at the airport at Continental.

To make a long story short, I checked with United, then Continental, and Continental didn’t show a ticket either. When I went back to United the agent apologized and said it was a “common problem these days”. Twenty minutes on hold later they had exchanged the ticket internally. (I heard later from the client that it was pandemonium at the airport.)

3. When checking in online, double check the flight times. Since airlines frequently change schedules these days, it’s easy to miss an update, and thus end up at the airport at the wrong time. If a flight has changed by a few minutes it’s not a big deal. But a more significant change could mean not missing the plane. (Or wasting a lot of time at the airport.)

4. No paper boarding pass? Get an electronic one. If a printer isn’t available at the time of check-in (even when you prefer obtaining a paper boarding pass at the airport) consider the mobile phone boarding pass option. Personally I am enough of a Luddite that I prefer having something printed. But, I’ve had a couple experiences where even the usually easy self-check kiosks had long lines, and being able to avoid them was a major advantage. In addition, passengers with the boarding pass sent to their phone still have the option of printing a paper version at the airport, which I usually do when I check luggage.

No doubt as check-in technology evolves there will be further tips, and if any readers have some additional tips now, please add them in comments. So far no major U.S. carrier has started charging passengers who don’t check in online. But, it wouldn’t surprise me to see that change in the not-too-distant future.