Hi,
Here is the one I found.

I sometimes fresh reinstall a N900, from time to time.
To restore it the "normal way", it's a long and sweaty way. Setup wifi, reinstall repos, rootsh, HAM, power-kernel, bootmenu, backupmenu, dependencies... all thorough ssh to make it easier, what means usb networking and so on. I never remember which wiki page is the right one and which files to modify for this...

And I don't mention the supplementary steps for repartitioning and modifying boot files to allow home and sd encryption...

As I am not doing it often enough to remember the easy way, I have to rediscover a not-to-hard way each time.
At the moment, I am serial reinstalling some N900, so, the systematic came back to my mind. I catch it before it goes away again and put it down here for later and others.
So this is my way,
What is yours?

My departure point is a backup from Backupmenu, with cssu and other apps and personal adjustments. You also must have a backup of your MyDocs, as it is not the same partition as /home (also called opt in backupmenu). It is another partition mounted in /home/user/MyDocs/.
Beware to reinstall the same kernel that the one you used when you made your backups. Here, I have the power53.

Here are detailed and basic instructions in case newbies still exist or will come on N900 - Dreaming is not forbidden

So:
1. Reflash your device (sure you backed-up everything?)
2. On a microSD or in MyDocs, copy your systemBackups dir with the backups from backupmenu and the following packages:
The ones with a * are not mandatory.

rootsh_1.8_all.deb
*zsh_4.3.2-25_armel_opt_a_installer.deb
tar-gnu_1.22-2maemo5_armel.deb
liblzo2-2_2.03-1maemo3_armel.deb
mtd-utils_20090606-1maemo1_armel.deb
libattr1_1%3a2.4.43-1_armel.deb
libacl1_2.2.47-2_armel.deb
i2c-tools_3.0.2-1maemo3_armel.deb
openssh-common_5.1p1-6.maemo5_armel.deb
openssh-server_5.1p1-6.maemo5_armel.deb
bootmenu_1.14_armel.deb
backupmenu_1.2.0-1_all.deb
kernel-power_2.6.28-10power53_armel.deb
kernel-power-modules_2.6.28-10power53_armel.deb
kernel-power-bootimg_2.6.28-10power53_armel.deb
kernel-power-flasher_2.6.28-10power53_armel.deb
kernel-power-settings_0.18_armel.deb
*fapman_0.7.1-1_armel.deb

Copy them after having them renamed to allow automating of reinstall:

000_rootsh_1.8_all.deb
00_zsh_4.3.2-25_armel_opt_a_installer.deb
01_tar-gnu_1.22-2maemo5_armel.deb
02_liblzo2-2_2.03-1maemo3_armel.deb
03_mtd-utils_20090606-1maemo1_armel.deb
04_libattr1_1%3a2.4.43-1_armel.deb
05_libacl1_2.2.47-2_armel.deb
06_i2c-tools_3.0.2-1maemo3_armel.deb
07_openssh-common_5.1p1-6.maemo5_armel.deb
08_openssh-server_5.1p1-6.maemo5_armel.deb
09_bootmenu_1.14_armel.deb
10_backupmenu_1.2.0-1_all.deb
11_kernel-power_2.6.28-10power53_armel.deb
12_kernel-power-modules_2.6.28-10power53_armel.deb
13_kernel-power-bootimg_2.6.28-10power53_armel.deb
15_kernel-power-flasher_2.6.28-10power53_armel.deb
16_kernel-power-settings_0.18_armel.deb
17_fapman_0.7.1-1_armel.deb

-On the N900, in the stock file manager, click on the rootsh package.
Application manager will start, let it install and quit it.
-Open a terminal, type root, you are now logged as root.
-Go into the directory you copied the packages
-type:
(the ">" are displayed and don't have to be typed)

With zsh, it would be
This should install all the necessary packages at once, freeing you to do all the sweaty things I mentioned before.

Then, reboot and you can restore your backups with Backupmenu and then MyDocs.

You should always download from a trusted source, like by searching here.
But if you trust, here are the files.

Have you found a better way?
I dreamed since 2010 to be able to make a custom image, once for all, that I could reflash from my PC but it seemed impossible to do. I am wrong?