As an effective SE, you need to build a good rapport with your salesperson. Before you think that this is too obvious and click away, remember that they will often say things that you will (hopefully inwardly) cringe at. In fact the Tech Demo Guy lists 3 things the Salesperson should never say while you demo.
At the end of the day though, going into a meeting, the more comfortable you are with the sales person the better you will sell together though. Build up an understanding of who will answer which kind of question, and have some simple subtle ways of cueing the right person to respond. This applies both in person and on webinars actually.
To build up this kind of relationship, try to engage your sales person with questions about the prospect, to make sure you understand what they want as well as they do. Expect the salesperson to ask you back about the contacts you have at the customer, and suggest action items after the meeting on how you can engage the customer to help the salesperson become successful. Remember as a Sales Engineer your job is to make the sales person successful. Its not enough to make the customer happy enough with the tool. The customer must believe your company can deliver it, so ensure there is no disconnect within the sales team.
Also, the sales person is the best positioned to see how well you do your job. Debrief after every meeting. Make sure they help you become a better SE - every customer meeting is a chance to become better.