I have taken part in many discussions on the XTension Home Automation Mailing List about the best way to wire electrical outlets for both safety and good propagation of X10 signals. The current favorite method is using some "pigtails" that are now showing up at hardware stores. They are a regular wire nut with a pigtail and crimped connector at the other end for screwing into the outlet connectors. They look like this:

There is some serious consensus amongst the folks on the list (few if any of whom are electricians and none of whom know anything about your local code or skill level working with these things so if you don't know about it keep your hands out of the electrical box. This article does not contain any information about and isn't meant to be a tutorial on electrical safety, either the immediate don't kill yourself or the more long term don't burn down your house sort of safety!) against using the "push-in" connections that are available on many switches and outlets. In my limited experience of replacing switches and outlets I have found these to be problematic at best and sometimes dangerous. Many times I have removed switches or outlets to find melting or evidence of arcing around these. There are also many documented cases of these connections causing attenuation problems for X10 signals. So if you're working on electrical parts of your house, replace and re-wire these things to get rid of them.

The other way you may see outlets wired is as a "pass through". There are 2 sets of screws for each connection on an outlet one on each side, so it is natural that some folks might connect the incoming power to one set, and then connect the other side to the wire running off to the next plug. However, this can place the entire load of the circuit on the internal wiring of the outlet and has also been linked to X10 troubles. Rumor has it that this is also against code in many places now!

The preferred method is by bringing the input and output wires together with a short length to serve as a pigtail. Wire nut those 3 together and then screw the pigtail onto the outlet. You can save a step with the pigtail by using these pre-made pigtails and people are talking a lot about them recently. They are considerably more expensive though.

Just for kicks the other day I cut some of the plastic off one to see how the pigtail was attached and I found that it's not really attached at all but rather relies on plastic tabs and the friction and pressure from the wires you screw up inside of it to hold the two parts together!

Here you see 4 outer bands that cradle the spring section. The spring section will pop right out after you've removed or deformed the plastic to this point.

This reveals the cradle is crimped to the pigtail and the spring just sits in the middle of it.

It also appears that the cradle might be brass or some other metal, meaning that you have dissimilar metals in the crimps and connections which aggravates corrosion which is a great filter of X10 signals. So this might get to be just as bad of a signal sucker in the future as having used some other method of connecting.

After looking at this I don't think these are worth the extra cost. I don't think they are dangerous or evil, but I'll just use a short length of regular wire and a wire nut in the future.