Getting around in the Netherlands is generally easy. The country is not big and the distances are small. A dense net of trains covers the countries and busses plough the smaller routes. Hitch hiking is another possibility.
The train is usually your best bet. Most routes have trains 2 or 4 times an hour, so you don't have to wait very long. Routes are organized by destination in the stations, which might confuse some vistiors. Train travel is kilometerwise not exactly cheap, but as the distances are not that big, it will not cost you too much. If you're going to stay for a longer time and will plan to travel around extensively, you might consider getting your self a 'NS voordeelurenkaart' for €49,- a year, or €39,- if you're lucky and there is some kind of action. Available at the stations. For the double price you get a card valid for two years, but this card also gives 50% discount in most museums. You need a bank account for that though. Use your international cell phone rentals in the Netherlands to make train reservations.
Busses are another option, but they are not very popular. Prices are the same for Interliner busses. Interlines are sort of long distance busses that cover some of the routes that have no trains. Prices are the same for Interliner busses. In summer you can get a zomer zwerf kaart, which will allow you to travel unlimited on busses for one day. It's a cheap but not very fast way of getting around. Use your Netherlands cellular phone rentals to check bus shedules.
Hitch hiking always has it's own risks and the rules are the same in the Netherlands. Avoid travelling alone, especially if you're female, don't step in a car if you don't feel comfortable. On the other hand, it can be an excellent way of getting around and talk to the locals. You'll notice though that the Netherlands are a very densely populated country, which makes the average distance a car travels much shorter then say in Germany. While you can get lucky, more often then not you'll find that the friendly driver that stopped for you will only take you for the next 25 kilometers.
Car hire is a reasonably cheap mode of transportation in the Netherlands. The roads are in perfect condition and you can rent a car for 1 week with all insurances and taxes from 199 Euro. Use your international mobile phone rentals to renta a car easily.