In order, beginning with the free things:
Clothes - wear the tightest fitting jersey you have, or even better a skintight base layer. Do not wear gloves. Some cheap lycra overshoes for £10 might come later.
Position - move the stem down as far as you can go, putting the spacers above it. You can also move the saddle forward (and maybe a tiny bit up) to help your more aggressive position. You should be able to have your hands on the hoods with your elbow angle between 90 and 110 degrees. MAKE SURE YOU NOTE YOUR ROAD DIMENSIONS BEFORE MOVING ANYTHING!
Tribars - a set of clip-on tribars can be bought new for less than £50, or used for as little as £10 on
ebay. Set them up properly and this will suddenly knock a huge chunk out of your time.
Helmet - an aero helmet such as Bell Meteor or Giro Advantage for around £100 in place of your road helmet will smooth the air around your head and offer more time savings. The head is the inconvenient bit on top of an un-aerodynamic mass, so anything to make the head and body appear as one continuous object is good.
After this unfortunately it becomes expensive for relatively small improvements....
Tyres & Tubes - rolling resistance is your enemy. Some fast rolling tyres such as Continental Supersonics, Veloflex Record, or Vittoria Open Corsa Evo CX paired with latex inner tubes will help, but will also degrade faster than stock tyres.
Carbon wheels - take your pick. The front is more important than the rear as it is cutting through clean air. 50-60mm Deep sections make a noticeable difference to any bike but at a huge cost.
Skinsuit - short of getting a full-on TT bike, this exhausts the upgrade possibilities. Prices range from £50 up to around £200
I'm a regular tester knocking out mid 21s on sporting courses with a full TT setup, and short 23s using a standard road bike, skinsuit and road helmet - 90 seconds difference for proper TT position, trispoke + disc, aero helmet.
