Looking more professional, a fashion thread

[JaredL]

I'm currently a fourth year grad student in Economics. Next year I'll be going "on the market." The market is basically a huge conference where people give talks on a variety of topics, some meetings take place, and most importantly pretty much all job interviews take place. In the fall schools post openings, the interviews take place at the aforementioned AEA meetings in early January. Candidates have anywhere from a couple of interviews up to 30 or more. If successful, a candidate is flown out to give an hour and a half talk at that school.

I don't spend a lot of time thinking about clothes. I pretty much always wear t-shirts, most of which are Oregon related. I have a North Face fleece that I have worn a lot this winter. I usually wear jeans or shorts when it isn't too cold. For shoes, I basically wear New Balance cross trainers.

I went on a bit of a binge last spring. Last Christmas, I took advantage of sales to buy a bunch of colored shirts, just your basic Old Navy/Gap etc. type. I also bought a few pairs of khakis and cargo pants, a pea (sp?) coat, and two pair of Clarks shoes (one brown, one black, otherwise identical) that I actually really like. I wore these pretty regularly for most of the winter and spring, but once summer came around and I wasn't around the department, I went back to wearing shorts and tshirts etc. For whatever reason, I haven't gone back to wearing the nicer clothes, and even some new stuff I bought early this month.

Basically, I would like to overall look more professional. I have the type of personality where I don't take myself too seriously, and the clothing I usually wear is representative of that. The problem is that I suspect it also rubs off and people get the (sadly not incorrect enough) impression that I don't take my work seriously either. Also, I think that when making the change from student to professor it seems appropriate to not look like a total slob all the time (at least until I get tenure). Some tips for day-to-day stuff would be good here. Also, I will need to buy a suit to wear to the interviews and hopefully fly-outs. I've never owned a suit and don't have the slightest clue what to look for. There is also usually a dinner which is somewhat, but not completely formal at these things.

So basically, I am seeking advice on 3 how to dress for three types of situations:
1. Day to day office - something nice and comfortable, should be professional but need not meet any dress code.
2. Formal suit/tie for a job interview, giving talks etc.
3. Something in the middle to wear to dinner and other events with potential employers.

I don't have a huge budget to buy tons of stuff. While I'm not going on one huge shopping spree, my hope is that I can keep reading this thread and make some improvements gradually. Any input appreciated.