I have worked for various companies with various policies. The best one was the simplest:

Give them X amount budget. Fixed. So long as they stay within the allotted amount, don't worry if they spent it on bubble gum or meals. Let them book their own hotels and flights and cars. So long as it is within budget, it should not be a concern.

You may think that using a travel service will standardize and reduce expenses, the truth is, this is not actually the case.

When I was given the freedom to book my own travels within my own budget limits, I tried to maximize return. This mean I became a great deal hunter...through things like Hotwire I could end up in a 5 star resort for 3 star money. I flew direct everywhere, rather than through 2 cities with tight connections merely to save 20 dollars. This meant a happier employee and better work.and I didn't burn out. Some times I even saved thousands. For example, I got a flight to the UK down 1900 dollars by going tuesday to tuesday rather than monday to friday. That freedom and flexibility saved the company money.

Going to a travel service means a computer basically picks the itinerary and hotels, it does not think creatively nor does it get the most practical deals. If I need to go from X to Y, it will pick what it thinks is the cheapest flight and route me through W and Z, making the trip take 6 hours longer. and often causing additional costs due to stupidly tight connections, missed flights, and lost work time. Travel services don't care if the employee had to run through DFW for a 30 minute connection..and all for a savings sometimes of less than 10 dollars. It sees that it's 10 less and books it robotically.

The other issue was using preferred companies. I worked for a company that struck a deal with Avis. We suddenly had to use it for all trips..the problem was that Avis, even with the corporate discount, was almost never the cheapest. When I had freedom to choose I could get a car for 75 a day..when the deal with Avis came along, that went up to an average of 110 a day. How they thought this saved them money was beyond me. So stay away from preferred vendors, as the discounts are a bit of an illusion.

Another incentive was the "rollover" budget. What I did not spend of my budget in a month was rolled over into the next month. This made me more willing to get those discounts and stay under budget, leaving me wiggle room later in the year if I needed it.

Increase those budgets for international trips to destinations where costs are higher. For example, Japan. If you cannot find a hotel for less than 200 per night, fixed budgets become a problem. 120 per night max may work for Des Moines, but not for Tokyo. Keep that in mind and include it in the policy.

Per diems as well. I think 50 to 60 is reasonable..but as I say, if you just have a fixed budget the employee can work it out. I never ate breakfast, usually had a small lunch (sandwich etc, under 10 dollars) and would get a nice dinner.

Many companies allow higher classes of travel for longer flights. C is reasonable on flights over 8 hours, at least for senior management.

Most importantly, if you are providing corporate credit cards, allow the employee to retain the rewards associated with them. I worked for a company that let us do this if we paid the company 75 dollars for the year. The benefit was that the card would collect miles and benefits that usually ended with us getting upgrades and free hotels, and some of us even used it for biz trips to stretch the budget.