~Some ideas for turning around a shy, scared, &/or overly reactive new dog~
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Odds are, when you agreed to adopt that rescue dog, you were expecting her to be grateful and to shower you with affection, huh? So why is she hiding behind the couch, growling if you approach or perhaps even streaking away in terror, spraying pee or poop as she runs?
Unusual as it may seem, this happens more often than you might think with rescued dogs who have had a rough past or were not properly socialized as pups. (Some breeds are more prone to this than others.)
Believe it or not, that "impossible" animal cowering in the corner is perfectly capable of becoming a "normal" companion...it just takes time, patience, and understanding on YOUR part. She needs to know that you don't mean to hurt her...and unlike some dogs, whose trust in humans comes naturally--your new arrival must learn that you are a friend who can be trusted. The older the dog is, the longer it will take...but rest assured that if you prove yourself to be safe, she will come around.
Here are some ideas on easing the transition for your scared or uncertain new friend. Disclaimer: This is NOT "professional advice", and no method or approach is appropriate for *every* dog—these are just things that have worked for me in the past.  If you are not well versed in dog language, you may want to have someone who IS, take a look at your dog and your interactions with her...just to be sure you are reading her messages clearly.  :-)  It's VERY important to be able to communicate with your dog! If you're not fluent in speaking Dog, now's a good time to start learning. :-)
Here is a page with some links to various sites I've found to be very useful when learning to think & speak like a dog: 
Required Reading

First things first: don't take offense. She's not doing it to spite you, or to be deliberately obstinate...I know that sounds like common sense, but a month later when you still can't get that leash on, you'll probably start to take it personally!

Don't rush things.
Whenever possible, try to let the animal set the pace.
You can't entirely forget about her, of course, but be careful not to push her to the point where she is obviously stressed out. Push a little bit...then back off, let her come to the realization that it didn't kill her to have a split second of contact. A day or two later, push a tiny bit further...then back off again. What you are doing, is very gradually "stretching" her limits and building up her tolerance level.
Understand that it may take a long time--sometimes 6 months to a year, for older, abused, or unsocialized animals--before you will be able to treat her like "a normal dog".

Never pull her out of her crate or hiding place...if she is frightened enough, this could trigger a fear bite. Also, don't chase her around the house or yard unless absolutely necessary. If you do need to catch her, corralling her into a small room where she really has no room to run (and closing the door) is the way to go. Do NOT hover over a cornered animal when approaching it! Again, that could trigger a fear bite. Instead, try your best to appear non-threatening. <See suggestions below on calming signals, and approaching a scared dog.>  You don't want to "force her hand" and stress her to the point where she does something you'll both regret, and you DON'T want her to get any more practice at reinforcing shy behaviour patterns.(!)
If you must leash her or pick her up, but feel you are at risk of a fear bite, you can cover her (or at least her head) with a blanket or towel. This prevents her from being able to bite you, and also has a calming effect on some dogs. If you find this does calm her down, you may want to cover her on occasion, then sit beside her and stroke or massage her gently...it's a less threatening way to build up her tolerance to being touched. Some dogs are also less sensitive to touch, when they are being rubbed through the blanket as a buffer.
You can also "herd her" into a crate or inside/outside with the blanket.  ;-) 
Again, be sure to read her body language: if you don't know what all the body, ear, & tail positions mean, look it up. You need to know what she's trying to say to you.

Let some things slide. Right now, your focus should be on getting her to trust you...not on obedience, housebreaking, or other less critical issues. You can get to those in due time ;)

If you don't already have it, I HIGHLY suggest the book "On Talking Terms With Dogs:
Calming Signals" by Turid Rugaas. Most canines prefer to avoid a confrontation whenever possible, and have an assortment of behaviours aimed at calming other dogs down. You can use these signals too, and they'll soon be second nature to you.
When you approach her (or try to entice her to approach you), make yourself small. Sit, or better yet lie on the ground.
Avoid looking directly at her: a direct stare is a threat or challenge. Turn your face away from her, avert your eyes, or at least soften the look in your eyes by partially lowering the lids when you look her way.
Blink at her (watch her! she'll blink back), lick your lips, pant (and smile while you pant). Yawn frequently and repeatedly. Move slowly.
Lie on your tummy, and paw at her like puppies do when they want to play. Crawl or slither slowly towards her while in this position...if she lets you get all the way up to her, slowly reach up to stroke or scratch her chest gently. Petting from beneath is less threatening than towering over her. So is being down on her level.
Try covering your head with your "paws" and whimpering, sometimes their curiosity will overwhelm them ;)
Don't overdo it...if she is still too frightened to let you near her, give it a rest and try again later. If you find yourself getting frustrated, anxious, or resentful, give it a rest. Most dogs can read you like a book, and if you are projecting negative emotions, all the calming signals in the world won't convince the dog otherwise. Think "gentle", "happy", and "at ease".