Since you're not playing a closed guard or a half guard, you're essentially making the choice to sacrifice control for the increased sweep and submission possibilities of open guards. So, its vital that you have sound gripping, framing, and posture. Gripping is especially important, since if you have no grips then there's little to stop your opponent from just running around your guard, bulling you over, or throwing your legs out of the way. Thus, your first move with an open guard should be to establish a forcefield around yourself and decisively grip anything that penetrates. You may also wish to close the distance and force your grips (I.e. "butt scooting"). But it's very important to be decisive with your grips (ex. "Double wrist grip, feet on hips, right hand to near ankle, right foot to RDLR hook" as opposed to "mad scramble to RDLR inversion")
Framing and posture are also important, since an open guard player will need good second-line defense. Your hips need to be engaged at all times (legs wide and bent, feet engaged), your arms need to be close in to your body to create kinetic frames, and your back should be straight and angled at a position that keeps you balanced. In other words, you should be ready to absorb a little pressure from your opponent and throw it back at him; if you're not properly framed and postured then you will get run through or run over.