I’ve already mentioned a technique called enrichment (or stuffing, as Anthony likes to call it). I use it all the time, mainly because of English prepositions: they are very supple and flexible, which makes them difficult to translate using French prepositions, which are a lot less pliable.
Example:
The people on the course will have to book their own hotels.
The preposition on can be translated by a French preposition (sur, à, le, etc.), but here, doing so would lead to an awkward sentence. It is thus more elegant to use a verb phrase:
Les personnes participant à la formation devront réserver elles-mêmes leur chambre d’hôtel.
Pronouns also often need to be enriched when translating into French:
You will endeavour to master all the concepts presented in the course. To achieve this, your full commitment will be required.
Vous tenterez de maîtriser tous les concepts présentés lors du cours. Pour atteindre cet objectif, vous devrez faire preuve d’un engagement sans faille.

Enrichment can also be necessary in some cases of subordination, especially when introducing indirect interrogative propositions.
The other decision you have to make is how you wish to organise your workload.
Vous devrez également décider de la manière dont vous désirez organiser votre travail.

However, enrichment isn’t the only way to solve this type of problem, as this example shows:
Autre décision à prendre : comment organiserez-vous votre travail ?
Here I used a change of punctuation, a modulation (change of point of view) and a change of syntax.
Writing this article and comparing the examples I gave made me realise that this technique might partly explain why a French translation is always longer than the English source text.