I use always HttpGet or HttpPost even when my action is executing a delete method on the database.
For what should I use then HttpDelete/HttpPut ?
If you build an OData service.
HTTP DELETE - Deletes the entity data that the specified resource represents. A payload is not present in the request or response messages.
HTTP PUT - Replaces existing entity data at the requested resource with new data that is supplied in the payload of the request message. (msdn)
There's a presentation with Scott Hanselman that might be interesting. (I haven't seen it yet.)
There's also a couple of lectures on pluralsight on OData if you have a subscription there.