Clip
brushes should be any every level you create if
you want the game play to be smooth running, the
slightest support beam or light fixture can cause
your player to get stuck against therefore causing
the game to be a stop and start affair.

As you
can see our small light fixture which is sticking
out just 8 units could cause the player to get
stuck so to fix this problem we need to add some
clip brushes.
A clip
brush is just an invisible wall which is not seen
in the game.
before
we go any further go to view>show>show
clip brush, this will make them visible
in the editor
okay,
load up the common texture set and locate the
small red texture called clip.
create
a small brush which is about 192 units high (we
create this height as we don't want the player to
jump on top of it :) and then with the brush
selected press E and this will allow us to drag
the edges around.
Once
you've pressed E, in the XYtop view drag the
bottom left blue dot so it looks like this.

Now
drag the top left dot until it's level with your
wall like so

You
can also place one over the top of the light
fixture so you clip brush will look like this.

Press
E to come out of the edge moving mode and repeat
the steps for the other side of the light fixture.
Other
Tricks
If you
have some open windows like you see in castles and
cathedrals it may be a good idea to place a clip
brush over the window to keep those pesky rocket
jumpers from getting to places where they
shouldn't :)
Effective
use of clip brushes
A level without any
clip brushes isn't a finished level.
Well that's the way i see it, there is nothing
worse than running around a fast action server
only to get stuck against a door frame, my
language when this has happened isn't repeatable
on the net :)
When I'm creating
levels I place clip brushes everywhere even places
you wouldn't think to place them :)

This
shot is from Q3DM1 and as you can
see it's covered in clip brushes which help keep
the level running smoothly.
Now your probably wondering why there are clip
brushes on the ceiling?, well this is to keep
those pesky rocket jumpers from getting to places
where they shouldn't :-)
That
best I've found is to place clip brushes along the
walls but only make them between 8-16 units think
otherwise the player will notice them when
playing the level.

Notice
the clip brushes running right along the walls and
the huge one covering the sky.
If
your door has a frame around it it's best to place
a clip brushes which starts level with the frame
and runs along the wall until it reaches another
wall or door frame.
Well
there isn't much more i can add to this but i must
stress the you should test test test you level
before releasing it, run along the walls and see
if you get stuck.
Hope
you enjoyed this.