1. Take the train or bus. Sounds crazy I know but look at it this way. Most transit centers offer free parking and a bus is about $2 each way so for a couple your at $8 total instead of $40 a night. The train could end up costing more especially if its for one night depending on where your going, but something fun and different. Tacoma to Seattle runs as low as $11 each way. I have also taken the train one way and bus back. I like to relax on the train and enjoy a beer, which is way better than battling traffic and parking issues.
2. Park somewhere else other than the hotel garage. You can't come and go as you please like with hotel parking but I think most of us just park and leave it. Many times a quick Google search will find you a web page or some price lists. Simply typing in for example Seattle parking garage prices will get you what you need. I have parked as little as one block away for half price.Typically for about $22 flat rate tax included you can find a spot. The hotels are charging at least $35 and up plus adding taxes to it. Its a rip off.
3. One last idea and it can be tricky but use find a meter spot or at least check for one. Just make sure you check the rules closely. I once mis read a sign which got me a $39 ticket. Might as well have paid for that hotel parking. But if you work it right, it can get you overnight parking for about $3 depending on when you are arriving and departing.
4. Finally: Always scour the sites when booking. I have found deals on Expedia that include free parking or I have booked deals directly with the hotel or Groupons that have included parking.
Never pay full price when its avoidable and no need to haul your luggage. I usually park out in the loading zone get everything unloaded and the wife watches it while I go park. Same on departing. I get the car and then load up. Its not that much of a hassle.