[Windows 10 Mobile] RustyGrom:
This is only confirmed working for the AT&T Lumia 1520 and may work as is for other phones but also might require tweaks to the XML (see below). It is very possible to brick your phone with this. Your mileage may vary!

From a high level this works by using FiddlerCore to intercept the traffic going to the Microsoft WPflights server that controls the Insider app and responds with our own custom data. The app accepts registry editing information from the web responses and acts upon that. This allows us to write stuff to select locations in the registry. In the case of the Windows 10 Preview, it appears to only look for your phone's PhoneManufacturerModelName to decide if it should be offered previews. Windows Update also checks this value. Other devices like Samsungs or HTCs may need different settings. My phone did revert to it's factory values after Windows 10 was installed (I actually think it might have done it after Windows 8.1 was installed but it continued with 10 nonetheless).

1.	For the Lumia 1520 (and other phones?) reset your phone using the Windows Phone recovery tool. I recommend not logging in and setting everything up yet and just waiting til the end and doing a reset and letting it restore after you’re on Windows 10. This may not be required for your device but most likely is due to the space issue cited by Microsoft. You can see your available space in the About menu of the Windows Insiders app. Before resetting mine was ~200mb and after it was ~600mb. You can try without resetting but may get an error after it tries to download and install the update.

2.	Install the [url=http://go.microsoft.com/fwlink/?LinkId=522524]Windows Insider app on your phone (you will need to have a store account configured)

3.	Open the insiders app and go to the about options in the app bar and screenshot/make notes/email the settings to yourself. You may need this later.

4.	Download and extract the WPInsidersHacks app (attached to this post) to your computer

5.	Start the WPInsiderHacks app on pc, approve any firewall requests

6.	Connect to the same WiFi that your PC is on. Edit the settings for the WiFi connection to turn on the Proxy and set it to use the PC’s IP address and enter 8877 for the port

7.	Open Internet Explorer on your phone and navigate to http://[pc.ip.add.ress]:8877 where [pc.ip.add.ress] is the IP address of the PC running the WPInsidersHacks app

8.	Tap on the link at the bottom to the fiddler root certificate

9.	When prompted click open and then install to trust the cert and ok at the confirmation

10.	Run the Windows Insider app and tap get preview builds

11.	Tap on which custom action you’d like to perform. In my case I hit "Set to ATT Lumia 635" and click the arrow at the bottom

12.	Accept the agreement and click the check mark at the bottom. the app will close

13.	Now clear your proxy settings and run the insider app again

14.	Tap get preview builds and login with your Microsoft account

15.	Select the fast branch (if you’re reading this of course you want the fast branch) and click the arrow at the bottom

16.	Go into the phone settings and check for updates. With any luck you should have Windows 10 there for you to install. On my AT&T 1520 it had to do 8.1 first but it kept going to 10 after 8.1 installed (had to check for updates again)

17.	You’ll probably want to go into the settings and do another reset and then log into the phone and let it restore your previous data

The Windows Insider options are populated from the WPFlights.xml file. Feel free to edit/add stuff. I've experimented with writing to other keys but had mixed results. I wasn't able to write to the Software\Microsoft\Settings\{1DEF9B7D-2322-40eb-A007-16A75D5CDA6F} for example (seemed to not like the curly braces?). It should have all the same permissons as the preview for developers app. You'll just need to run the WPInsiderHacks app on your PC and point your phone's proxy settings to it if you want to go back and set something again (such as enabling tethering).