Strength is handy. Regardless of armour type. Lift weights. Find any excuse you can to be physical. Whilst it is true that the average medieval man was fitter than the average man of today. Today our elite performers are vastly superior to our ancestors. Modern physical training methods work. Go to the gym, lift weights, and you'll fight longer, recover faster. And for you Sir Ulrich, eat more!
Breathing is important as well. If you breath using your traps, pecs, and neck muscles you'll be lifting all your torso armour as well as your ribcage. If you belly breath you're only pushing out the armour that covers your belly. The less you breath, the less energy you burn. The less energy you burn, the longer you can fight before you run out of energy. Being in the right state of mind leads to efficient breathing. An example of western breath control can be found in the prayer before combat. Prayers are often spoken at a controlled pace, this regulates breathing. Think of the English army at Agincourt, in the morning they prep, advance, pause and pray, attack... win!
Here's an article on how to make a CoP
http://www.armourarchive.org/essays/cop_pod/. If you have any fears about measurements, you could make a CoP out of cheap fabric and plastic/cardboard. Then cut your steel and leather using the cheaper material as a template. Here's one CoP template
http://www.armourarchive.org/patterns/wisby_cop/. There's quite a few more different styles in the 1361 Battle of Wisby book. Some style have larger plates, some smaller, some have shoulders, some don't. Have a google.
http://www.onlinemetals.com/merchant.cfm?pid=9900&step=4&showunits=inches&id=839&top_cat=849 is an American company that sells sheet steel, they're selling 36" x 48" 16 gauge mild steel for $44 USD. Not necessarily the cheapest. If you hunt around you'll probably find a place that will cut it for you as well. Most cutting places use vector artwork, you can create this using Inkscape (free from
http://inkscape.org) to trace around your pattern.
Have you considered lamellar? What makes it good is that you can lace them together yourself. It is faster to make than mail, and is better at reducing blunt trauma - though not as good as CoPs or larger plate harnesses. See
http://www.polarbearforge.com/lamellar.htm for an example of various lame designs and an indication of cost per square metre. I think those prices are a little high and that you can get them cheaper by going directly to a steel cutting company.
Please note I haven't used any of the commercial companies above, the links are just there to give you an indication of what is possible. Shop around!
