Frame aerodynamics;
Here is about the only comparative study I've seen of different frames;
P3C = 690g of drag for a size 56 bike
Trek TTX = 690
P2C = 705g
Pinarelo = 705
Felt DA = 725
QR Lucero = 735
Kestrel Airfoil = 760
Kuota Kaliber = 780
Guru Crono = 790
Scott Plasma = 800
Orbea Oro = 810
The person
The entire bike system only represents about 20% of the total drag, the person represents the other 80% at racing speeds. For this reason, getting a good, low drag position is key. This is why a TT bike is really rather good.
Accessories
You may have heard the old myth about a time trial helmet is more important than aero wheels? Ignore that as it only holds true when there's no wind.
If you've got a TT specific steed, you can have dedicated tt bars which is nice and lets you shift while stretched out. As other have said, TT bars are the best improvement. Most importantly, get used to the position over the winter as the more aero you get, the lower the power you put up. You need to find the combination that gives the greatest watts/grams drag for you.
Get a deep section up front (a tri-spoke 2nd hand of e-bay is the best bang per buck) and an aero rear later if you can afford it. numbers wise, changing from a Kysirium front to a Zipp 808 front (arguably the most aero non-disk) will save you just under 90 secs over 25 miles. A disk rear will probably save you roughly another 30-40 secs.
A TT lid is good, but make sure you're using it properly; an aero lid with the tail sticking up can have higher drag than a regular helmet. Ideally, the tail should be flat against your back. TT lids should be tight and ideally, add as little frontal area to you as possible. Also, tape up the vents and you'll go faster.
A skinsuit will give a small improvement, so it's worth doing at some point. The same can be said of overshoes.
Conclusion
What Blah said
Longer conclusion
If you're going to end up with a full TT bike at some point, start off with a TT frame and bars to get used to outputting power in the correct position. I'd also add a helmet so that you can get used to the neck cranked back position. Use it over the winter on the turbo. Deal with wheels, skinsuits, overshoes etc, later. This is because they will not affect your power or anything about your riding style, just reduce overall drag.