Don't necessarily go for the cheapest. Make sure they have a good grasp of English over e-mail, and be clear about what costs are and aren't included. The tour company isn't your guide. But the guide will be your company, so never assume that all the correspondence has been passed on to the person in front of you.
Always check when it's appropriate to do so, and share what is important to you so they understand from the start. If you're after more culture than history, say so. If photo ops are your thing, ask them to take you to the most picturesque locations.
Once they know your specific interest it's a lot easier for them to pull you aside (if in a group) or point you in a certain direction.
Ask ahead
The best guides I have ever had informed me "two steps ahead". "Tonight we're meeting here for dinner and then you have your own time, and we're checking out at x o'clock and can then change your money at y."
Asking ahead will ease any planning anxiety and give you a sense of freedom knowing how much time you have before you need to be somewhere.
Surprises are never good. This goes both ways. Make sure you're communicating with your guide as much as you can from Day 1. Depending on your level of OCD-ness and if you speak the local language, this can be a fine line. However, if something seems at odds with what you were expecting, politely but firmly inquire.
Once on tour don't forget that you're the client and your guide genuinely wants to do the best for you, so be as up front as possible.
Be generous
Guides, by nature, tend to be curious about the rest of the world. Whether it's showing them selfies on your phone or the wonders of the Remarkables, be generous with your time in telling them about New Zealand. Everyone is surprised we have so many sheep. And hobbits.
Buy the gumboots
It's a metaphor for, "The reason your guide is suggesting something might not be the reason you suspect."
This is where you have to trust your guide. In my case I had perfectly adequate tramping shoes and gaiters for the jungle; but I should have bought the gumboots he recommended. I had to take my shoes on and off at every house and I became a muddy, time-consuming, embarrassing mess.
Make the most of it
Like all occupations, sometimes you get a guide who is a bit of a dick. You'll spot them; they're more interested in their own comfort than worried about your needs; they tend not to translate unless you ask; and are very poor at letting you know what is going on.
All you can really do is continue to push and ask questions, and reach out to others you meet along the way to find out what you'd like to know. Thankfully these guides are the exception.
Above all, remember that your guide is just that, someone to help, assist and recommend. All going well you'll be wishing they were available for every trip you go on.
