Don't get overly familiar when texting, or do it too long without meeting, as it can create a kind of "fake boyfriend" situation where you're acting like you know each other. When you do meet, you might discover he's not for you, and then it's awkward to "end" the non-relationship.
Don't use a jokey style when texting, or make hints; be as clear and unambiguous as possible to avoid misunderstandings. People on OLD come from the full spectrum of jobs, education, political beliefs and social settings and you can't assume they're on the same wavelength. You can show them your GSOH when you meet and can judge them better.
Go to a date knowing how you are going to end the evening if you a) like him, b) need another date or c) are not interested. Just a simple phrase like "It's been lovely meeting you but I guess we're pretty different/there doesn't seem much of a spark so I don't want to waste your time" / "How did you feel about it - would you like to try another date?" / "I've had a great time, text me if you're interested in another date".
Many people have multiple early dates, it doesn't say anything about you as they don't actually know you. In fact, if you're up for it, going on multiple dates yourself can be a good way of ensuring that you keep your options open and are not as bothered if someone doesn't want to meet again.
Watch out for your safety - don't accept lifts or meet for strong drinks at first. Plan your route home, meet for a quick coffee "on your way to meet friends".
There are plenty of liars on there, claiming to be unmarried/separated when this is simply not the case. Signs of this include living a long way away from you (so you won't pop in unexpectedly), there being some reason why you can't visit their house, them not answering calls in your presence or only being able to meet at certain times. Sure, it could all be innocent, but keep an eye open and use condoms.