Markie,Optimally, you should not force the sound. That means you shouldn't be experienceing tension in your neck and other places. Mouthpiece pressure should be enough to create a seal so air doesn't leak out the sides of the mouth.
1)Read Mouthpiece Pressure Assessment
2)Watch Urban Agnas videos on "Flow"
If you notice that you are using too much pressure, do this.
1)Put the mouthpiece in the horn and then, remove the tuning slide.
2)Then, hold the horn lightly and buzz a single soft tone.
3)Once you establish a good soft tone that doesn't quiver and require tension(which should be easy), begin to apply a little mouthpiece pressure.
4)What you should notice is the pitch moving up with just the application of a little pressure. The purpose of this is to show just how easy it is to use mouthpiece pressure to raise the pitch.
5)Now, buzz the tone again and this time instead of using mouthpiece pressure to raise the pitch, use the corners of the lips to raise the pitch.
What it appreas you are doing is using the wrong muscles to play.
To fix this, do lip slurs using the 7 valve positions
Do each position for about a minute (an honest minute) and be very aware of the muscles you use. Be sure to use the corners of the lips and not mouthpiece pressure.
Will this exercise kick your butt? Oh my yes!! Your lips will feel like they just ran a marathon. Why? You're learning to use the correct muscles to play instead of strong arming the horn. Play the slurs light and soft! Never loud. Lip slurs are like push ups. Now that you know what muscles to use Pay close attention to not fall back to using the strong arm method.
Here's the up side:
If you honestly do these 7 lip slurs every day ( make it fun, do bugle calls like revelle, taps, make sure to go as high and as low as you can on each valve) by the time Christmas gets here, this problem will be in your past and you'll be back to being happy
Pay close attention to Urban when he talks about posture, relaxing and how to breath.
When Christmas get here, give us a shout. A great gift would be to here that you're on the right path and you're having fun again.
I have found this to be the single best exercise of my development. It "is" teaching me what I need to do with my embrochure and using the corners. It is also teaching how to use my air and aperature together. This is the best advice yet!