Additional crystal malt will help add more caramel flavors. Say 40L.
I've heard that carastan (a type of caramel malt, about 38L) has more toffee type flavors than regular crystal 40, but I haven't noticed this myself the two times I've used it.
I've seen some recipes that call for taking a gallon of wort and boiling it down to a pint or so (then adding it back into the main boil) to achieve a more intense malty caramelly type flavor. I think some scottish ale recipes call for this. I've never done it myself, though.
Kegged: #114 Oatmeal Stout, #115 American Amber
Planned: APA, IPA, Nut Brown