France, Belgium and the Netherlands
In France, Belgium and the Netherlands church membership is a private matter with no tax implications. EU directives on personal information oblige the church to make a note of the wishes of anyone who would like to leave. A court has ruled that under the French privacy law one has the right to have one's name deleted from the baptismal record, but a Catholic bishop has launched an appeal.
Although almost a third of the Dutch have been baptised Catholic, Dutch bishops have told the Pope that their church is facing "collapse". In 2011 the government issued a report saying that thousands of children suffered sexual abuse in Dutch Catholic institutions and that Church officials knew about it but failed to stop it or help the victims. 
Benedict XVI's Christmas message in 2012 didn't help, either. Many Dutch Catholics were unhappy that the Pope used the occasion to attack gay marriage. In large numbers they immediately visited a website with instructions on how to leave the Dutch Catholic Church  So many have done this, in fact, that in 2013 the Dutch Catholic Church said it was planning to close 600-700 churches over the next five years. 
In the Netherlands this turns out to be easy to do: just send in three copies of a letter stating your wish to leave the church, accompanied by your ID. 
However, when the copies sent to the parish of Father Harm Schilder he took action. He announced that he would display the ID photographs in the entrance hallway of the church so that everyone could see who was trying to leave. 
In France and Belgium church-leaving is also easily done. Here the Church has been grudgingly forced to abide by the EU directives on protection of personal data (issued in 1995 and amended in 2003). This, and a French law from 1978, mean that you have a right to access any record about you, (whether electronic or on paper) and also the right to have it corrected, with the record-holder obliged to provide you a copy of the amended record (or proof of deletion).
However, in France a court ruled that emending it (with a note in the margin) is not enough: it must be deleted. On 6 October 2011 the French appeal court of the Manche Department in Normandy ruled that in accordance with Article 9 of the Civil Code, the church must erase the record of any baptism, if asked, because this is a record which is accessible to third parties and therefore falls under French privacy laws. 
The first Frenchman to go to court to get his name deleted sees it as “a sort of honesty toward the church because they have a guy on their register who doesn’t believe in God.” However, the Church doesn't appear to be interested in his integrity. Fearful that this could set a precedent, the bishop has launched an appeal.  He argues that the baptism does not fall under the French privacy laws because it is not a private act, taking place in a church which is a public place and that, furthermore, it isn't even an act of any kind by an infant being baptised. 
For France template letter for registering this wish, which cites part of the 1978 law, is here (in French).
For registering the wish to leave the Catholic Church in Flanders, there is a site which translates as "debaptism" which offers help (in Flemish) for submitting the required information to the Church by email. And for doing so in francophone Belgium and Brussels, this site (in French) offers downloadable forms which it will even send in to the church authorities. However, it warns about the Bishop of Liège (formerly a prince-bishop) who makes the special demand that everthing be done by letter (no email or fax) and if confirmation is desired requires a stamped self-addressed envelope ― otherwise he will administer a rebuke and demand a service charge of €7.
1. "Dutch Catholic sex abuse revealed", Press Association, 2011-12-16. http://www.guardian.co.uk/uk/feedarticle/9999268
2. "Catholics research leaving church after pope's comments on gays", Irish Times, 2012-12-28.
3. "Dutch bishops tell Pope the Church is collapsing as they face hundreds of closures", Life Site News, 2013-12-05. http://www.lifesitenews.com/news/dutch-bishops-tell-pope-the-church-is-collapsing-as-they-face-hundreds-of-c
4. http://www.ontdopen.nl/ "Ontdopen" means "de-baptise".
5. "Dutch priest to display photos of Church quitters", AFP, 2013-01-09. http://www.thejournal.ie/dutch-priest-photos-quitters-747457-Jan2013/
6. "René Lebouvier, premier débaptisé de France", Le Figaro, 2011-11-18. http://www.lefigaro.fr/actualite-france/2011/11/18/01016-20111118ARTFIG00632-rene-lebouvier-premier-debaptise-de-france.php
7. "Europeans 'De-Baptize' In Growing Numbers, Church Officials Worried", Religion News Service, 2012-01-18. http://www.huffingtonpost.com/2012/01/18/europeans-de-baptize-church_n_1214256.html
8. "Débaptisation : l'évêque de Coutances en appel d'une décision du tribunal", Belgicatho, 2011-10-28
"Recours en 'débaptisation' devant la Cour d'appel de Caen", La lettre de droit des religions, No 46, juin 2012.
2012-05-31, p. 16-17 http://www.droitdesreligions.net/ldr_pdf/La%20lettre%20du%20droit%20des%20religions_06_2012.pdf