I've not come across any tutorials that focus directly or indirectly on Star Trek photoshopping issues (which is a crying shame let me tell you lol).
So I've written something quickly up below which should prove helpful (works for me).
I've assumed in this tutorial that you're aware of the basic tools and functions of PS (and that you have PS ). If you need additional directions just let me know.
Illustrated ship is the Intrepid Class rendered for "Exodus" created by Flat Eric.
(Ship without nacelle glow: http://img351.imageshack.us/img351/394/1uv9.png )
1. Create a new layer on top of the layer with the ship.
2. Set that layer to linear dodge.
3. Set the foreground colour to a bright blue (you might want to enlist the help of the eyedropper and create a brighter variant blue from the nacelle)
4. Set the brush size to about double the width of the nacelle. Set the opacity between 70 and 80%, hardness should be 0%.
5. Paint over the nacelle. If you have trouble getting it to go in a straight line what I would suggest is rotate the ship until the nacelles are directly horizontal, and then go to View>Show>Grid, and then tick View>Snap. Once the grid is up, align the nacelle with a grid line, then grab your brush and paint it again, the paintbrush should then stick to the grid line and the nacelle.
The effect should look like this: http://img19.imageshack.us/img19/6316/2au7.png
6. You can either leave it as it is if you're happy with it, or you can create a larger more obvious glow. Simply get a larger brush, keeping the colour as it is. Reset the opacity to a lower level (judgement call again) and paint over the nacelle once again. You'll get a brighter glow that stands out more.
This is an illustration with a bit more working on it: http://img501.imageshack.us/img501/6289/3es5.png
This same method can be used for anything that glows. Hope it helps.
Also a great place for tutorials on all sorts of issues (some good space ones too) is http://www.good-tutorials.com though there are a few broken links.