What do I mean? One thing that will set of alarms in your girl’s head is when you start treating her differently with no explanation. There are usually two ways that you’ll start acting differently. You’ll either become the nice guy, or the mean guy.
The nice guy.
Don’t start doing overly nice things for your girlfriend if you haven’t before. She will become highly suspicious of you. If you haven’t ever spontaneously bought her flowers, don’t start now. Same goes for buying her gifts, taking her on trips, and being romantic.
When some guys cheat they tend to feel guilty about it and that manifests itself in the form of being nice to their girl. She will notice and become suspicious and she will start to dig. Don’t do it.
The mean guy.
Some guys get mean when they’re cheating. They get very defensive when confronted even with innocent questions. For example if you’ve been out hooking up after work and you come home and your girlfriend asks where you were and you get defensive and fire back at her with “why do you want to know”, or something like that, she will instantly be suspicious. Just tell her you had to drop by the post office/store/run an errand/etc. Just don’t make your lie too big or something where she would ask followup questions. Don’t tell her you went to see a friend in the hospital or something like that because she’s definitely want more details.
Basically what you want to do is treat your girl the same way you always have. Be who you always have been.