Trekking in Laos questions

Hi there. having been through exactly what you describe, let me tell you.

July/August in Laos is low season. There is no reason to book in advance, just get off the bus and find a clean place. The places that take reservations are generally the more expensive ones.

You also should have no problems booking treks once you get there. Here are two treks that should give you an idea of what you'll find.
Trekking around Luang Prabang

This one I did outside Vientiane. I would Highly suggest going to a national park (as this one was)--
Trekking through the Laotian rainforest

You can go on a non national park trek, but you;ll likely only be in jungle where it is too steep to grow anything. I prefer the national park journey. Do watch your step, I came awfully close to stepping on a green viper.

Are you sure you want to trek in the height of the rainy season? Definitely it's cheaper to book once your there, i.e. as a 'walk-in' customer. You can stroll down the street comparing the offerings of the various agencies. Most travellers don't book things like treks in advance.

In Luang nam Tha, it is very easy (and much,much cheaper) to book when you are there. There are quite a few trekking agencies lining the main street with boards out the front offering their treks. The more people to sign up to a trek the lower the price gets.
You can arrive one day, a be trekking the next day and your guesthouse will store your luggage.