-"Hell is paved with good intentions"
The road to hell is paved with good intentions: It means good thoughts or intentions are not enough to get you to go to heaven.  It's good actions that count.

-"Pain in the neck"
Just an idiomatic saying which means that someone/something is annoying.
You are a real pain in my neck.  You are a royal pain in my butt (neck can be replaced by butt.)

-"Feast or Famine"
It's either feast or famine.  It's either nothing at all or too much.  No inbetween.
You either have too much of something or you have too little.

-"Waste not, want not"
Those who waste will want. Those who save will have.

-Don't throw caution to the wind.
Don't take big risks.

-Lightning never strikes twice in the same spot
It's unlikely that a bad situation will be repeated in the same place or manner.

-In like Flynn
Dates back to 1945, refering to how easily movie star Errol Flynn could get women into bed with him. http://www.worldwidewords.org/qa/qa-inl1.htm

-Labor of love
Labor you undertake because of pleasure in the work itself, not because someone forced you to do it or you expect to get paid.

-Uphill Battle
You are fighting your enemy and there are other additional factors that make the situation disadvantageous for you.

Can you please provide examples using "feast or famine", and "waste not, want not"?
Try Goole or any of your other favorite search engines.  I will leave this up for you to finish.

-Pull the wool over his eyes
The wool refers to a powdered wig. To pull the wool down over a man's eyes is to temporarily blind him to deceive him.

from http://idioms.thefreedictionary.com/pull+the+wool+over+eyes
to deceive someone in order to prevent them from knowing what you are really doing. Don't let insurance companies pull the wool over your eyes - ask for a list of all the hidden charges.

-Now you are cooking with gas
http://thingsihate.org/article/626/things_i_dont_hear_anymore: "Now you're cooking with gas!"
Indicating that someone, after initial failure, is now employing an efficacious idea or procedure. I think this comes from a radio advertising campaign for natural gas, circa 1930s-40s.
Usage: When a friend has been having no luck removing a bicycle wheel, but suddenly finds that turning the wrench the other way works better, supportively exclaim, "Now you're cooking with gas!"

-You can't live with them, and you can't live without them
A woman might say something like "men, you can't live with them and you can't live without them."  She might say it in a humorous manner to mean that a man can be very handy and helpful, but at the same time cause a great deal of grief!

-You are what you eat.
If you eat a lot of healthy food, then you will be fit and trim.  But if you eat a lot of junky, fatty food then your body will become fat and unhealthy.  You are what you eat, so take care to eat good food.

-Go to the dogs
Don't know, sorry.

-Down in the mouth
Don't know this one either...maybe someone else can help.