Getting a stagiaire

There is a special contract and tax regime for trainees, called 'immersion professionnelle'. This type of traineeship is originally thought to be offered to young graduates, but you can basically hire anyone you want, also from other EU countries.
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The traineeship agreement (note that it is not legally a work contract) has to define the objective of the traineeship (what is supposed to be learned), the place and length of the traineeship, working hours, amount of the internship allowance, the rules concerning the ending of the contract, trainee's tasks and responsibilities and the name of the supervisor. Your payroll agency will be able to offer you with a model contract.
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The length of the traineeship is not explicitly defined by the legislation, but in general the acceptable length is considered to be 3-12 months. The traineeship agreement can not be renewed or prolonged. The students are not allowed to be in internship for the same employer for more than 60 days during the academic year.
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You'll have to make the DIMONA declaration of the trainees in the same way as of employees.
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The interns work the same hours as other employees in the company, but there are restrictions concerning the evening, night and weekend work. Note also that the interns are not allowed to work alone (be alone in the office).
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You need to make a separate risk assessment of the health and safety risks related to the traineeship (special protection of young employees, young referring not to the age, but experience).
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The employer pays the trainee an allowance of minimum 736.20EUR (for 21+ year old, amount is indexed yearly). There is a tiny contribution for social security, but otherwise both the employer and the trainee are exempted from the taxes and social security payments.
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The trainee will be covered by your normal compulsory employer insurance (accidents at work) and you'll have to pay a small premium of him/her. Check with your insurance agency what is the procedure of informing them about the intern.
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Other EU nationals than Belgians will have to register in the commune upon arrival to Belgium / start of the traineeship. The traineeship agreement should be considered as sufficient proof of ability to support yourself and of the social security cover.
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You'll find further details including the minimum allowances from here: http://www.emploi.belgique.be/defaultTab.aspx?id=3460