Since SharpDevelop 4 you should already be familiar with the key shortcut F6 or the context menu entry "Find derived symbols" on members and types:
When called on a type, it shows a popup with all types deriving from it (or for interfaces: implementing that interface) and allows to navigate to them. When called on a virtual or interface member, a popup with overriding or implementing members will appear:
With the new SharpDevelop 5 RC the same is possible in the opposite direction: "Find base symbols" (previously "Find base classes") is now working on class members, too:
Called on a virtual/override member it will show base members:
The same is possible for interface members, that have been implemented by selected member:
The new shortcut Ctrl+F6 can be used as well. This feature should make it much easier to navigate in your project.