Let's start at the base, with a good ol' primer. The Benefit That Gal Brightening Face Primer does what it says on the tin. This pinky-hued cream can be worn patted over makeup as a subtle highlight, but I prefer the effect given when I wear it under foundation. It adds a subtle sheen to the face (not a bad one!) and the colour of it brightens any dullness you may be suffering from. Plus, it's a primer, so it smooths everything out and keeps your makeup on longer - bonus!
If that ain't enough for you (or, y'know, this could be an either/or situation) then my next suggestion would be the coveted YSL Touche Eclat Highlighting Pen. Granted, there are much cheaper dupes around, but this is the only highlighting pen I own and so it's taking center stage! This is great for applying under your eyes in the infamous Kim K triangle, over the cheekbones, down the nose, anywhere you'd like to brighten. The glow given is subtle and it's a step I feel lost without now!
Of course there are the dewy foundations and the shimmery highlighters (my favourites are Rimmel Match Perfection and Sleek's Highlight from their Face Form Contour Kit in Fair, respectively) but when I'm needing that extra boost, these are the guys I reach for first.