I think the key is to add lots of water, while pureeing it. If you are boiling the veggies, use the water that you used to boil, it's full of nutrients (as long as you washed everything good enough). I think squash and potatoes, especially in the beginning you reallly have to use more water than you think. I remember thinking I was just watering everything down too much, but in the early stages, it helps alot. Plus if you're freezing then defrosting, the water will kind of separate a little so if it's too much you can scoop it off the top.
Also, with the squash, what kind are you using? I think butternut is the best, most nutrients per scoop, acorn comes next. Don't use the spaghetti squash yet...it's too stringy.
Another thing is that I really overcooked those hard veggies a bit. I think the squash is a lot softer and less stringy if it's a little overbaked!