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Wow! Those are some handsome petunias! The last photo has buds--the long tubular things will open up the next day into petunias.
I've never dead-headed--the spent blossoms just fall off on their own.
If you are getting fewer flowers, you may want to fertilize gently with a fertilizer that has a higher middle number (phosphorus) which helps flowers and roots grow. Since this is a container plant, I personally would use half the recommended dosage. There is no "leakage" to surrounding soil in a container, and salts/fertilizer/trace minerals can build up in the soil. A full dosage could "overfeed" the plant and cause some burning of roots.

 

I wonder if it's getting too much water? Do you just water everyday... even if it the soil still feels moist? Mine are in the blazing sun, I only give them a drink every 2 days. They are pretty things, aren't they?

 

I know petunias are heavy feeders and sometimes they just get tired. My growing season is SO short that about halfway through I have to cut all my petunias back (they look ugly for awhile) then they come back just beautiful :-D

 

I went and took a few pictures for you of deadheading on my petunia in a planter. Do you pull the whole petiole off or just the flower? If you are leaving the stem that the flowers grows from and there is a seed pod maturing the plant will spend a lot of it's energy maturing seeds rather than setting new flower buds


deadheading ( photo / image / picture from carolyn keiper's Garden )
This is the whole part that needs to be removed. as you can see in the last pinching i broke off part of the new growth...oops, but not a big worry.





deadheaded petunia ( photo / image / picture from carolyn keiper's Garden )

This is what the stem should look like after you have removed the spent flowers.







seed pod/petioles ( photo / image / picture from carolyn keiper's Garden )
This is the petiole. the whole pod and stem is removed.

( photo / image / picture from carolyn keiper's Garden )
Here I have torn some of the petiole off so you can see the seed pod better.

Have you been fertilizing the plant weekly, also? they are heavy feeders and do need to eat, just like us. Hopefully this is the information you are looking for. Sometimes without pictures it is hard to explain exactly the answer. I use a lot of fertilizer on my baskets. I think there is a brand called "Jacks" that makes a fertilizer specifically for petunias.

 

All this reminds me that I haven't fed any of my potted plants in a while. I'd better do it this evening when I get home from work!

 

Elishasmama, there are slow release fertilizers (Osmacote is one type) that fertilize plants over a long period of time. I haven't used them, but probably someone here on the Stew can advise you if a slow-release would be beneficial, and if so, which one is best for your petunias.
Next we are going to introduce you to the wonderful world of roses!

 

I need to feed everything in my yard too~quite a chore!
I did spray my geranium babies and a few petunias with a foliage feed, liquid kelp

 

Elishasmama, not all plants need fertilizer. In fact, most herbs are at their best in "lean" soil, which means it isn't terribly fertile. I put my herb garden where the soil is the poorest, but has good drainage and don't fertilize, so they develop the oils that give them their taste.
Flowers, in general, benefit from fertilizer. I fertilize gently about two weeks after transplanting in the spring, or after the seedlings emerge. Then in the case of those that bloom more than once, I fertilize again after each flush of bloom. We have a lot of native plants, and those I only give a small feeding in the spring, since they have adapted to our conditions and don't need much encouragement.
Vegetables, I feed regularly. A feeding when they emerge or are transplanted, again when they start to blossom, and again when they set fruit.
We use composted cow manure, and composted chicken manure, so dosages are determined by how the plant looks, and by a semi-educated guess! You seem to be doing well with your petunias, and now you know how to dead-head them, and how to fertilize (ask your neighbor what she used--gardeners just love to be asked about things like that). Then branch out to other plants.
Not all roses are thorny. We have a Clytemnestra climber (apricot blooms) that has very, very few thorns. There are others out there that are almost or entirely thornless. NEVER buy a rose from Wal-Mart--always go to a local nursery who can help you if you run into problems.