One trick the change agents try to pull is take your Dollar bill, start counting your Bolivars, change the Dollar bill for a fake one (without you seeing it) and then cancel the transaction and give you the fake Dollar bill. Make clear to them that you are not stupid. Don't be intimidated by them and ask different agents for their best deal.
Most tour agencies, hotels, and hostels allow you to transfer money from a European or American bank account and they will give you the equivalent in Bolivars. (Although it can be a lot more difficult for people accounts in the UK.)
== Cities ==