Very timid

how long have you had her? if she's new, perhaps she's still adjusting.

Is this dog simply shy (like a timid puppy in the dog park - running behind his person's legs) or is this dog actually afraid (hiding, shaking, submissively urinating etc.)? 
 
I would say, if this is a puppy and he/she is just a little shy, do some confidence builing type work - starting with some sort of positive reinforcement obedience work (maybe clicker training) where you will teach very basic stuff and reward with treats and praise for every step int he right direction, so the dog learns that he/she is good and that he/she is learning and pleasing you!  If you go really slowly, don't get frustrated and treat and praise for every little thing, your dog will probably soon forget to be shy and concentrate on working with you!
 
If, this dog is actually terrified, hiding, shaking etc. and you have not had the dog too long (if you have had him/her a long time, and this is a sudden change, I am not sure what to tell you), I have read that the best thing you can do is wait it out...  Let the dog come to you.  Do not overwhelm the dog with affection and do not play into the fear, you will only teach the dog that fear is desirable.  Have treats at the ready - when the dog does come to you, or come out from the corner or under the table - don't make a huge deal, just praise and treat, quietly.  Let the dog adjust and build confidence at his/her own pace at first.  Then once he/she has come out a little bit, start doing some work to build confidence.
 
I think, wither way, the most important thing you can do is NOT coddle the dog or play to the fear, you will only teach the dog that the fear is good and that you are happy with that behavior.  Instead, make sure you reward any small steps toward confidence. 

I think a bit more info is necessary.  Some dogs are naturally timid, or at least seem to us to be timid because they are cautious.  What breed is your dog?  How old?  How long have you had her?  Under what circumstances is she timid?  Does her tail ever wag?  Is she eating regularly?  (sometimes what we interpret as "timid" or "sad" behavior can be a medically related problem.) 
 
There's so many possibilities from health related issues to environmental adjustment issues.  Make sure your pup has regular visits to the vet, is up to date on her shots, and has a regular, consistent schedule (food at the same times every day, potty breaks at the same time)  Dogs thrive on schedules and it can throw them off if their schedule is altered often.
 
Hope this helps