- ICONS. This seems like such a small thing, but you need icons for both stores. Windows 8 is fairly self explanatory, but for windows phones, use iconic tiles. Update your tiles in the WMAppManifest.
- USE THE CERTIFICATION KIT. Both Windows 8 and the Windows Phone SDKs for Visual Studio come with Ceritification Kits. These are very useful but time consuming tools that will make sure your apps are technically compliant. Pain in the rear, but can save you frustration down the line.
- HAVE WINDOWS 8 PRO. Though you don't need windows 8 pro to develop the apps, you DO need it to test your apps and deploy them. If you don't have windows 8 at all, you're looking at $200. If you have windows 8 basic, it's about $100 upgrade.
- Licenses are a thing! Keep in mind you will be spending $50 for a windows 8 license, and $100 for a windows phone license!
- Incorporate Back button functionality in your phone apps. Though construct 2 doesn't do this natively, there's a couple of great plugins that not only do this, but will give you support for in app purchases, trial mode, and more. Plugins are AWESOME!
- Don't check more than "America" in regions you're releasing to. Okay, that's not technically true, but be aware that in certain countries, Windows Market will require you get certified by the local ratings board to release to that market. For example, Australia and Brazil require it of their games.
I hope you find my advice helpful, and that it saves you some frustrations!