1) Do some hands-on if at all possible - Rather than just taking about something, try to show it too
2) Don't make your slides too fancy - It is easier to read big black text on a white background. If your presentation is being recorded, plain slides are much more readable on the recording.
3) Try to test your laptop out with the presentation system before hand. Don't wait mere minutes before it is to start to discover that the projector doesn't like your default resolution. This is especially important if you are using someone else's laptop are have not used it to give a presentation before.
4) If you are at a Linux conference, for goodness sakes... use Linux as the OS on the machine you are presenting with. If it is a loaner laptop and you don't have any choice, mention that to your audience so they won't assume you don't like Linux on the desktop. If it is not Linux specific but "open source / free software" oriented, try to use free software. Macs are for elitist smucks. If you are using both Windows and Microsoft Powerpoint for your presentation, make sure to apologize and if possible, have some gifts for your question asking audience.