A quick tip for new users (probably most of you already know).

If you install large packages, the Maemo watchdog has the bad habit of kicking in due to install processes taking too much of the N900's resources and making it unresponsive. This will reboot the N900, interrupting the package installation, and most likely breaking the EasyDebian image.

1. It's always a good idea to backup your working EasyDebian image.
2. When you install software from the repos, use apt-get install and not synaptic.
3. Prior to installing, remove any external USB storage, and unmount the microSD card manually. For the latter, as root do:
umount /media/mmc1/
4. Use the following command from Debian Chroot:
nice -n 19 ionice -c 3 apt-get install --no-install-recommends <package name>

A little explanation:
Unmounting external storage is to prevent a potential issue with dpkg (used in the backend by apt-get install) which seems to have caused reboots for some.

The nice and ionice commands are to avoid the install processes from taking too many resources, making the N900 unresponsive, and causing the watchdog to reboot.

Also, the --no-install-recommends flag for apt-get install is necessaty in squeeze and later because otherwise it will consider recommended packages as dependencies, causing a lot of space from our limited Easy Debian images to be wasted.

Finally, to remark that if you want to be extra cautious you can additionally install only a limited number of packages at a time to minimize even further the chances for a reboot.

This last strategy is implemented by first finding out every package that will be installed (including dependencies) without actually installing it, for example by using the -s flag of apt-get install, as in:
apt-get install -s <package name>

Then, just install a subset of the packages set at a time.

Thanks to leetnoob, sulu, qole, and others for the original tips. Just condensing some of what I've recently learned by going through the thread.