In general, you need to start building your own class libraries. Class libraries can be part of multiple projects and changes to those libraries will be reflected in all your projects. If you want certain parts of your libraries to only be changed in one project make a copy of the library and keep it separate.

IOW, you can have a library that contains only form classes, or only custom controls, or whatever.

You should also look into creating your own framework if you are planning to develop multiple projects that have common components.

Data should be kept project specific unless you are developing a large modular system with multiple applications that use a common database.

HTH