You should find an operator or broker close to your place in order to find the best deals. Smaller brokers usually are the best.
I recommend:
For transatlantic flights from Switzerland (and nearby):
Albinati Aeronautics
For intra-Europe short and cheap flights:
Home
For intra-Europe + UK longer and more comfortable flights:
Private Jet Charter Europe and Management of your aircraft
In the US-Caribbean:
Book a Private Jet Today - Fly XO - Private Jet Rentals;
Helidosa Aviation Group
Avoid NetJets.
I forgot to mention that those 25-50-100 hours jet cards (possibly refundable and non expiring) are a good and safe way to park cash if you don’t know what to do with it, or for other reasons…
Another tip: most big companies have one or more planes available for charter at good prices. Sometimes these companies also have their own aviation company (for obvious tax reasons).