Just install AlwaysLFG, and type "/join LookingForGroup" if you haven't already. From then on, AlwaysLFG will make sure that you're signed in to the LookingForGroup channel whenever possible.
What exactly does AlwaysLFG do?
WoW requires you to be in an LFG queue in order to have access the LFG channel, but it's often nice to be able to see LFG messages even when you're not specifically looking for a group.
AlwaysLFG solves this problem by automatically adding you to the LFG queue for the first item in the "Zone" category, which is typically unused. AlwaysLFG will add you to this queue (a) when you first log in, (b) when you leave a group, and (c) when you type "/alfg join".
As a result, just about the only time you'll be out of the LFG channel is when you're in a group that is not using the LFM tool--and in that case, there's not much you can do about it anyway.
By the way, you can also type "/alfg leave" if you decide you'd like to leave the LFG channel.
How do I customize the LFG comment?
By default, AlwaysLFG sets your LFG comment to say that you're just listening to the LFG channel, so that people who see you in the LFG tool know that you're not actually looking for a group for that zone.
If you'd like to change this comment, type "/alfg config" and enter a new comment. You can set up separate profiles for different characters if you'd like to have a different LFG comment for different characters.