<p>I'm developing an Eclipse plug-in, based on a bunch of core Eclipse plug-ins like SWT, JDT, GEF and others. </p>

<p>I need my plug-in to be compatible with Eclipse 3.3, since many potential customers are still using it. However, personally I like the new features in Eclipse 3.4 and would like to use it for my development. This means I need PDE to reference 3.3 code and, when debug, execute a 3.3 instance.</p>

<p>Any tips on how this can be achieved?</p>

<p>Thanks.</p>

You can change the 'Target platform' setting to point to the location of an existing set of eclipse 3.3 plugins. This will compile your code against the 3.3 plugins, making sure that they stay compatible no matter which version of eclipse you are using to develop the application.
The setting is under Window->Preferences->Plug-in development->Target Platform