The quickest fix I personally found was giving them candy or fruit from the store - it's like praising them but only stronger. But whatever she does when you give her the candy, she will likely be doing more often.

Other than praising (green hand will help), I found that picking up collectables helped a lot over the long run.

If some parent has a promotion or there's a new baby in the house, it should help her happiness too.

But happiness will go up slowly. Give it some time, it will get better as time goes by - of course you have to make sure there's enough food in the fridge and she has no underlying illness (if she has been sick a lot, consult a doctor). Keeping the house tidy and the lawn free of weeds (as much as possible) is also a must.

Hope that helps, that seems to work for me anyways.