How to lighten pack?

Does anyone have any tips or tricks to lighten up packs? My pack is pretty darn heavy, and I'd love to have some tips to make it lighter. All I can really think of is getting a bivvy sack instead of a tent and getting a smaller sleeping bag... what do you guys do to make yours lighter?

Both are excellent ideas.
You want a breathable bivy. I use the Mountain Hardware Conduit SL, which breathes amazingly well in a snow cave or under a tarp, but wouldn't be suitable if directly exposed to the weather. (It will take some searching to find a good fit for your needs.)
There are mass sleeping bags out there that way next to nothing. Most of them are really expensive, but they don't have to be. Camp out at the Bellingham REI Garage sale with your current stuff, and book it to the sleeping bags as soon as it opens to find a good one for not a lot of $. It will also take up less room. The tents are a killer for weight, especially the cheap family-camping type. A hammock might be suitable for SAR, so long as you are always around trees.
Also, I think Thermarest products to be much too bulky, weighty, and expensive. Go with a cheap $10 gray close-cell foam pad, it works just as well and you don't have to worry about tearing it to shreds on Course 1.
A light stove is very nice, but can be expensive. Look around, or share one with someone else.
I have also cut down on extra clothing, other than socks. I wear the same stuff every day because it dries really quickly and I don't care if I stink out there. I think the extra just adds unnecessary weight to my pack.
I only carry two liters of water and purification tabs. That is a personal preference thing, I know it's bad, but I only drink that much in two days while I'm hiking, and there is not need to carry extra water weight once you've measured your consumption under stress.
Food is power bars, a cliff bar for breakfast, two sandwiches, meatballs, and an orange. This is the only place I don't consider weight. Oranges taste soo good out there, and they take more abuse than an apple, so to me it's worth it. Personal preference thing, I guess. Italians don't skimp on food.
My pack itself is a tradeoff between weight and durability. I think its a gregory advent pro, which is light, but touchy on sharp edges and when you store ropes internally. It is much lighter than my previous pack, though and I love it.
I carry a flint/magnesium lighter + knife for all my fire/stove-starting needs.
I can't think of anything else.

Its hard to say without knowing what you pack, but just think about everything and ask yourself if you really need it. If you don't use something often, then leave it out. On searches I stopped carrying overnight gear because I have never spent a night sleeping in the field, if at all possible, just keep searching. Bivy sacks are good, but if its going to rain then you might need a tarp or waterproof sheet of some kind to hang over you. Also, unless its really cold you can just sleep in the bivy with no sleeping bag, and stay reasonably warm for an hour or two, which should be enough time on a real search. When you get cold, start searching again. Remember though, that you need to have enough stuff to warm a hypothermic person.

Yeah, the sharing thing is important. If you know that you are going to be with someone, split up things that you dont need two of. On a search two years ago we knew that the person was a few miles up a trail and Conner, Phil, and myself threw what we needed into one pack and traded off carrying it. We got to him about 20 minutes before anyone else, doing it that way doesn't leave much margin for error, but in some cases it works.
About the water, I would say never carry more than 2 liters. I drink a lot, so I bring iodine and fill up a few times a day if possible. Drink as much as you can on your way to the search (at least one or two liters); if you start hiking dehydrated, you will not rehydrate.

I will echo both Dan and Michael.
Lots of xtra clothes are great if you are car camping but just take the necessities on SAR missions. Trainings are the best places to figure out what you need and dont need. Try packing light on Course 2 and see exactly how little you can get by on THEN plan for the contingency of a patient.
The only time I carry a tent of any sort (or even a hammock) or my thermorest is on trainings because I want that added level of comfort.
When we did a weekend survival exercise out in the woods with nothing but what we could stuff into our vests, thats when I realized how much crap I was carrying that I had no need for.
Dan pointed out a good point about sleeping in the field. On the rare occasion that we have to do that it is going to be a few hours of poor sleep taking care of a patient or just trying to make it to daybreak.
Making a pack lighter doesnt mean going out and buying a bunch of more expensive gear.

All that being said, is it safe then to really only pack extra socks or should I pack maybe one extra undershirt and pants in case I get wet?

I only pack extra socks and a light set of poly-pro. If needed i can throw the poly-pro on any patient. I usually stay dry when it rains and with my gear i dont really have to worry about being drenched with sweat. I have a very light tent, and if i need to worry about weight i can substitue that out and just use my bivy and like joel stated i can handle being uncomfortable for a few hours.

Edit-

Nvm figured it out.