I have a hard time finding technical clothing that fits properly. I'm 5'11" and last weighed in under 140, yikes. For casual clothes, medium is fine. When climbing I need something a little tighter fitting. If I go smaller in size I will start to lose length. I am looking to replace a Marmot windbreaker and a Columbia down jacket, both medium. My wife and I can fit in the Columbia, zipped. That doesn't sound like a medium!
Are there any brands that typically run slim? Often times small isn't even available in stores or online, particularly for sale gear.

You might have some luck buying your clothing for the length and then taking it to your local dry cleaner to alter it for you. My local dry cleaner is great at altering clothing. Not too exspensive either usually a couple bucks covers it.

I was about your size at one point and still am very slim and have always fit well into the more athletic cut Patagonia clothing.

I thought of that the other day, but I'm concerned they won't know what to do with technical gear. I don't want to leak water or down fill. Probably just needless paranoia.

I'll throw in a shameless plug for Arc'Teryx... Most of their stuff is geared towards climbing endeavors whether it be rock, ice, or alpine.

One of the few companies that fits my skinny 6'1" 140 lb frame. Expensive as hell but bomber construction.

I second the Arcteryx recommendation - the only mass produced clothing company that fits for tall & skinny. Aside from fit, I also think their gear incorporates some of the most thought out and practical designs on the market. Pricey, yes, but worth it if you take into account how long you'll own the gear and how much better it is than the competition.

Some of the smaller Euro manufacturers also make great fitting clothing for tall and skinny folks, but most of them don't export to the US (but if you ever find yourself in a euro mountain shop, you'll be amazed at how most stuff there just fits. Designing for a different market, I guess).

Good luck.

I've tried the eat as much as I can and sit on my arse diet with no results.

I'll third the Arcteryx claim. The gamma mx pants come in a medium/tall size, perfect for my 6' 2" frame. My understanding is that the origional owner of Arcteryx was a tall skinny guy and desighned the clothing with that in mind.

Never had much luck with Patagonia. It seems to be made for short,barrel chested euro dudes. I dont qualify in either department.

Ever looked at beyondclothing? They custom fit most of their clothing based on your desires and measurements. Expensive though.