Swimming with a straight back

I don't have any specific advise for you.  I just find this statement, in isolation, funny.  

Build up that core strength! It takes some work from your core to hold the correct swimming position.

(you can build this in and out of the water - through s/b/r and through core strength training)

I imagine stretching out further and reaching further ahead of me.  I guess it's mental for me but it helps. 

As the above poster mentioned, I think you probably need to work on your head position (looking down, not out in front). I've been swimming forever, but somehow developed that "head up" habit in the 20+ years between age group and master's swim--probably from mostly swimming in crowded pools or open water where I tended to look ahead all the time to avoid obstacles. What really helped me was doing kick sets, esp. freestyle, with flippers, arms at sides or out in front, head down in the water (no board). Try looking ahead, then looking down. You can really feel the effects of bad vs. good head position on how cleanly and quickly you glide thru the water.

It also helps if you wear a swim cap-- I find that both with myself and the kids I coach, esp. girls, hair, even fairly short hair often gets into eyes and encourages a swimmer to look forward and that possibly leads to arching the back a little. I've never heard of squeezing the butt--in fact, I can't say I've ever thought about my butt in the water!

First, I am curious if you have a gymnastic background using the term "hollow" back.  I had never heard it until my daughter was heavy into gymnastics.

I created a straight line with tape on the carpet in our workout room.  I then practice streamlining on my side on the tape.  This is where you are on your right side, right arm straight out in front of you, left arm on top of your side with your fingers extending towards your knee.  It is the position right before rotating and doing a pull with your right arm.

When I first started really swimming my instructor asked me to get into this position on the side of the pool.  So I thought I did.  He indicated, no, I want you to be straight.  Wow!  I thought I was and he was full of it until I got home and put the tape down.  

Over the next 1+ year and doing a lot of swimming I now can lay on the tape and be perfectly straight.  Well on one side.  I have a problem on the other side.  Plus I also now swim straight. 

I am not a swimming expert.  But I don't believe you want your back completely straight.  I think you want it hunched some.  Hunched but still able to be straight on the line I explained above.  You will need a fish to help you with that aspect.