Well, what type of gaming channel are you? Lets player, tutorials, how-tos, previews, reviews?
Make your trailer about your channel and specifically what type of content viewers will see if they subscribe.
Have a call to action VERY early on as they can skip after 5 seconds.
Ensure you grab the attention somehow in those first 5 seconds.
My trailer (check my sig) as an example, states "Are you into indie gaming?" as the first thing. This tells the viewer that my channel is based on indie gaming and I've received a LOT of positive feedback regarding the trailer. It then features clips from previews I've done, the main series on my channel that brings the biggest viewership.