Flash games store your progress in .sol files, which behave similar to site cookies. This often means it's quite easy to lose them or see them deleted. This page contains some tips on where to find the flash save files for NULLL games and how to copy these files.

The location is slightly different for each computer, the general location looks as follows

Windows XP :
C:\Documents and Settings\[username]\Application Data\Macromedia\Flash Player\#SharedObjects\[xxxxxx]\[website]\#nulllgames\saves

Windows Vista + Windows 7 :
C:\Users\[username]\AppData\Roaming\Macromedia\Flash Player\#SharedObjects\[xxxxxx]\[website]\#nulllgames\saves

For windows computers in general, click and hold the windows key on your keyboard and press r and box will pop up, in that box type : %appdata%, then press 'OK'.
This will take you the the Application Data folder for windows XP, or the Roaming folder for vista/Win 7. In that folder then go to :
Macromedia\Flash Player\#SharedObjects\[xxxxxx]\[website]\#nulllgames\saves

Mac :
~/Library/Preferences/Macromedia/Flash Player/#SharedObjects/[xxxxxx]/[website]/#nulllgames/saves

On Linux or Unix :
~/.macromedia/Flash_Player/#SharedObjects/[xxxxxx]/[website]/#nulllgames/saves

In the locations above, [username] is different for each windows installation - it's usually the name of the person who installed the operating system. [xxxxxx] is a bunch of random numbers and letters, again different for each computer.

The [website] part of the location depends on which site you are playing the game on. If an identical game is played on different sites, two savegames will be created.
For the NULLL Games website this part is nulll-void.com
For Kongregate this is chat.kongregate.com
For Newgrounds this is uploads.ungrounded.net

If you are using Google Chrome as your browser, your savegames can also be located here :
C:\Documents and Settings\[username]\Local Settings\Application Data\Google\Chrome\User Data\Default\Pepper Data\Shockwave Flash\WritableRoot\#SharedObjects\[xxxxxx]\[website]\#nulllgames\saves

If you are playing an AIR version of a NULLL Game (i.e. a version that needs to be installed), the location will be different for each game.
For HD Xyth :

Windows :
Replace [username] with your windows username
C:\Documents and Settings\[username]\Application Data\HDXyth\Local Store\#SharedObjects\#nulllgames\saves
Or copy and paste the following :
C:\Documents and Settings\%username%\Application Data\HDXyth\Local Store\#SharedObjects\#nulllgames\saves

Mac :
~/Library/Preferences/HDXyth/Local Store/#SharedObjects/#nulllgames/saves

This save file is identical to the one from the flash version of HD Xyth, so you can replace it with a savefile from any flash version of the game or vice versa.

All NULLL Java games will place their savegames into the default user folder for the operating system you are using. Inside that folder, each game has its own folder where the savegame(s) for that game are located.

Windows :
C:\Users\[username]\NULLLsaves

By default, Android will place savefiles into a seperate folder for each game. This folder can't be reached unless you have full root access. Uninstalling the game will also cause this folder and thus any savefiles in it, to be deleted.

Once you have found the location above, you should be in the 'saves' folder, which can contain the savegames for multiple NULLL Games (except for AIR versions, where each game has a unique folder). The names of the savefile per savegame are as follows :
HD Xyth : HDX00.sol
Crystal Chaos : CC01.sol
Fleet Bounce : FBnc01.sol
Rivi'i Hell : RvHell.sol
Technospire 2 : TS2sav.sol
Hydranought : HydraSave.sol

If you are unable to find the location, you can try doing a search on your computer for the filenames instead.

As mentioned above, if you play the same game on different sites, your savefiles will be placed in different locations - these locations can't communicate with each other, so your progress is not automatically transferred between the two sites.
If you are playing the game on the same site, but from different computers, your progress is also not automatically transferred between those computers.
In either case, if you want to move progress from one site to another or from one computer to another, you'll have to copy and paste the savefiles.

Browsers treat flash savefiles as cookies and thus can easily delete them. Updating your browser, clearing cookies or even using privacy protection add-ons or browsing in private/incognito mode, can all result in the deletion of your flash games.
Check your browser's settings and if needed, add exclusion rules to your privacy protection add-ons for those site you play flash games on.
Before updating your browser, it's a good idea to backup your savefiles as well. An update sometimes include deletion of cookies (Firefox is known to do this).

Using the info above, you can locate your savefiles and copy them to another location for safekeeping.