As mentioned, increase fermentables. Generally, also increase the fermentability of the wort, to keep the beer dry. And add more hops. But these are general things. Just doing them blindly (without analyzing what the results will be), can be a recipe for disappointment. Software/spreadsheets and Recipe Critique Posts can help.
^^^ what they said ^^^ Brewing a 'Double' IPA, for example, is not as simple as doubling everything. As you gain experience (including your own mistakes), you'll learn how to Imperialize pretty much anything.
But how can I imperialize my session beer without raising the ABV or changing the flavor, color, or ingredients?
