Dell laptop power cable always falls out!

Duct tape fixes everything.

Properly, though? If you're adventurous and handy with a soldering iron, try replacing the internal power jack. You should be able to find a service manual for your model (via google) that describes the proper way to open the case.

Barring that, you'd have to talk to Dell about getting it fixed.

I guess that either the laptop power port or the end of the cable have worn and widened. Same happened with the S-video port on my Dell 1525. Depending on whether it's the cable or the port (and on mine I'm quite sure that it is the port), you'll probably need to get creative. If the cable- gently squeezing with pliers (careful, own risk, etc.) to make it a bit elliptical so it grips at two points. If the port (or the cable) try to widen the connection (duct tape? too thick probably, maybe something from the medicine kit).

If the laptop is stationary, then you can of course tape or glue the cable to the machine.

A new one shouldn't be too expensive..?

Cut 1mm from the width of brown tape, wrap it round the nearest end of the connector, cut off any excess. Should add that extra bit more to make it a tighter fit.