My method is slightly different from some of the others here as I generally us a lower temp for most of my smoke and then bring it up for that last couple hours. The brisket turns out beautiful.
Wild swings and spikes may have a great effect on the meat but those who are using an electric smoker shouldn't be experiencing this problem, so let's look at some of the basics.
One thing my buddy and i do for sure is test the meat before we buy it. I have trimmed, I have purchased pretrimmed, I have cut, now I leave it as is. I enjoy the flavor that all that extra fat adds to the meat. The only thing we check for when purchasing a brisket is how far it will fold. I grab it by the ends and try to fold it. If it doesn't fold I put it back and test another one.
How are you cutting into it? Are you cutting with the grain or against it? This will make a huge difference in how tender it is.
Are you wrapping it in foil? If you are having a hard time getting the tenderness you like or trying get it to be more juicy, using foil can help. Having it wrap good will hold all the drippings which will cause steam to occur, this acts kind of like a pressure cooker. Leaving it wrapped when you remove it and allow it to rest, it will continue to cook and steam itself for a while. Once it begins to cool it will draw some of those juices back in.
I have never tried injecting or using tenderizers but one might look into it if they absolutely can not achieve the results they are looking for. However, the basics should never be skipped.
I hope something here helps someone out there. Now, could someone open one of these threads for beef ribs? I just can not seem to nail great results trying to smoke them.