Hi
I prefer to plan
Edit: trips following the exact course of the road so that I can see contours, etc. I like BikeHike for this because it's quick, easy and you have Google maps (with satellite view if necessary) and OS maps simultaneously displayed. It will also give you elevation data if you choose that option.
I have the Etrex 30 and don't use plotted
routes because the Etrex warned me that "only 50 points can be used for on-road navigation", which was always fewer than I'd plotted in BikeHike with my preferred "follow road" option selected (843 on a 46 mile route).
BikeHike has a trackpoint reduction option (under "options" at bottom right of the Mapping Controls window) if you want to reduce the points count to 50, however I don't use this.
I now use tracks because they aren't restricted to 50 points. These give a line on a digital map following the course of the road, however, you don't get the "turn" warnings. You could liken it to following a paper map, one that isn't soaking wet and blowing in the wind. And you don't have to stop to refold it.
Tracks are just what I want. I zoom the map between 200 and 800 feet depending on the complexity of the road layout.
I plot tracks using BikeHike and save them as a
gpx track to my computer. I keep a copy there (there is an option on the Etrex to delete your data that I mistakenly used once

) and also copy them to a micro SD card that fits into the Etrex (beneath the batteries).
I plug the micro SD into my computer directly for saving/copying files because it's much faster than going through the Etrex. As Lolly Kat said, you'll need to set up a Garmin folder with a GPX sub-folder in (on?) the micro SD to save your tracks to (the Garmin unit has a similar folder setup for reference purposes).
You can also save downloaded maps to the Garmin folder in the micro SD, again much quicker if it's plugged directly into your computer.
You can get both full-size SDcard and USB adaptors for micro SD cards depending what ports your computer has available.
Edit: you don't have to use the micro SD option, the internal Etrex folders do the same job but there's less storage space
HTH Regards
tim-b