Wilfred Laurier

The thing about 'better' parts, is that when your 'lesser' parts are new and working properly, you get practically zero benefit from more expensive parts - your bike will be exactly the same speed with Sora shift levers as with Dura Ace Di2. There might be a tiny increase in drag in the bushings in the derailleur jockey wheels, but not much when they are clean and new. There might be a tiny increase in drag in the bottom bracket bearings, but you'd be hard pressed to find a measurable difference at 100rpm. Upgraded crankarms and chainrings will not make you one bit faster. Same with front derailleurs and brakes. More expensive doesn't mean faster. There might be a tiny tiny weight difference, but if you get to your destination with 100ml of water left in your bottle than you could have saved more weight there than by swapping 105 for Dura Ace.
SO make sure your equipment is working at its peak - keep your hubs (which I believe are cup-and-cone and not sealed cartridge bearings) lubricated and properly adjusted. Buy better tires* and keep them inflated to a pressure that prevents pinch flats but doesn't bounce you out of the saddle over rough pavement..

*Tires: There is likely some improvement to be found there, but a fun game I like to play is to only buy new tires when my current ones are worn out. I treat it as buying myself a reward for riding so much I wore out my tires (the rear one, anyway). My favourite tires are Panaracer Paselas - they have a very lightweight casing that rolls super fast, are available in every size under the sun, and are affordable. But everyone has their favourites and say they are the best. The more you ride over the years the more types of tires you will get to try.

Other free speed: ensure your bike fit is optimised for your body - there are a lot of suggestions above that you lower ('slam') your stem, but if this puts you in a position that makes your back or neck hurt, or you have a belly that compresses your lungs when in a low aero position, then you will not be able to put out as much power. If your saddle is too low then your body will be working ~10% harder to put the same power to the pedals as with a properly adjusted saddle.
Also wear tight fitting clothes - all the slammed stems and deep dish wheels in the world won't make up for a loose windbreaker flapping in the wind.
If you are carrying luggage, get an aerodynamic or seatpost mounted rack - do not wear a backpack or use side-mount panniers.

Milton Keynes

I can't believe that nobody's suggested painting the bike red.

Gresp15C

Ride with other people who are at or slightly above your level of ability. I'm nowhere close to being competitive on any bike, but I know that I make a better effort when I'm riding with others.

wphamilton

Taking the question literally look into high performance tires, latex tubes, or better yet change to tubeless. In that case, needing a new wheelset, look at deep section aero rims. Gains will be marginal.