Trains are expensive, but there's a good deal to be had. For 76 euros if your 26+ and for 51 euro if you're 26-, you can get a Rail Pass (it's called: GO PASS), which gives you 10 rides from any train station to any other train station in Belgium. You can share this ticket with several people. You can also use this ticket for your traject inside Belgium when traveling abroad but you need to write down the last station before the border (even if your train does not stop there), which means you'll have to buy a ticket from that last station to your destination station abroad. For example, when going from Brussels to Amsterdam (and not taking the Thalys) you can buy a ticket from Essen (B) to Amsterdam and write down "Brussel" and "Essen" on your Rail Pass.
Hitchhiking is quite doable. Check Hitchwiki.
European Carpooling (Taxi Stop - Eurostop)
Taxi Stop is an agency that offers carpooling possibilities to drivers in destination or from European countries as well as passengers looking for a destination. The website is available in English (as opposed to its German equivalent) but requires to create a free subscription account in matter to contact a driver or a passenger. Although the availability and destination listing does not require to log in. This service is called eurostop and has a set rule regarding the fees which is pretty simple: registration to the service is free of charge, a passenger is simply asked to pay 3€/100km to the driver once at destination. The concept of carpooling is so popular that people might come to you when you hitchhike and offer you a lift. Always clearly say in the beginning that you don't want to pay (assuming you don't) so that there are no misunderstandings.
Hotels are expensive, but you can easily find hosts through hospitality exchange. Free camping is quite hard in the Flandres. In de south of Belgium (the Belgian Ardennes and 'De Hoge Venen') are more accessible for wildcamping. All land is mostly private domain, but it's easy to not be seen.
Supermarkets are slightly more expensive than in the Netherlands and Germany. Aldi and Liddle are the cheap shops. There are also a lot of bio organic and nature food shops. (Bruges, Ghent, Antwerp, Brussels...)
You can find relatively cheap prepaid SIM cards with data.
Mobile Vikings is one of the best deals for data. 15 euros gets you 2 GB for 1 month, 1000 SMS'es, 1 hour/day free calling to other MVs and you can still use the 15 euro for calling. Internet abroad in the EU is 15 cent per MB.
Belgium can be really good for busking. Part of euro zone and quite wealthy country, the coins you get are usually worthy enough to be saved as an income (traveling cheap with no money for accomodation trasport and food).
All touristic cities downtowns are excellent places and usually police don't bother you too much.
Brussels is the capital of Belgium and of the EU. Many countries have embassies in this city. A lot of countries don't have embassies in neighboring countries such as the Netherlands and Luxembourg, so if you're a resident of these countries Brussels can sometimes be a convenient option for acquiring visas.