I got tricked last time when I rented a car by being assigned a similar car and not the POLO I wanted. I even chose the Polo in the list and not the Opel Corsa I got in the end. What should I do not to be conned next time? To clarify, I want the POLO not the upgrade to a Megane or some other car similar to GOLF.
You can't guarantee it. However, you can increase your chances by showing up earlier, before they start to run out of cars (at midnight most of the cars are gone and won't be back until tomorrow afternoon and evening), and by being a frequent renter with them. Also, try asking. I have had rental agencies give me the "good news" that I was getting a bigger car and saying "please, I don't want a bigger car, they are hard to park, I am by myself and don't need extra room" has restored my smaller car to me.
It would also help not to call what they are doing a con. Many people are happy to get a big car for the price of a small one. They always give you something that normally costs more. If you don't happen to like it, I get it, I feel the same, but it's not a trick or a con. You and the clerk want the same thing - for you to leave happy and recommending them to others. Work with the clerk not against them. They don't know you don't want the upgrade. Most people do.
And if all else fails and you haven't prepaid, pull out your cell phone while standing at the counter and start calling other places to see if they have the car you want. Be clear you don't mean "or similar" you want that exact model. If they do, off you go and try to get it. If they don't, well at least you'll be making an informed decision about whether to accept the upgrade.
If you read more carefully when you booked, you would almost certainly have seen the like "VW Polo or similar" with the "or similar" often in much smaller text.
As for what you can do, you can book with Hertz and pay extra to use their "book this exact model" option. There may be other hire companies offering this.
In general car hire companies will advertise more desirable cars, then give you a Vauxhall/Opel. My experience is that at least in the large hatchback or smaller categories, you are very unlikley to get a VW - the cheaper ranges in the VAG group (Seat and Skoda) sometimes, but rarely a VW, never something premium like BMW or Audi without paying extra. However some companies do allow a "special requests" or similar text box when booking. I've found that expressly requesting not a Vauxhall works to avoid being lumped with corsas.
Overall, it's good planning when booking a hire car to ensure that the "worst" car in that category would suit your needs. I've had trouble in the past with needing storage space, seeing "Polo or similar" and thinking that would be fine, to be given a smaller Opel or Citroen. I've learned to book for larger when needing the space etc.
Make sure that the contract you sign either has a tick box for this exact car, and also put that in the comments box. If booking by telephone, make sure that the operator understands that you want that make and model, and will accept no substitutes.
I don't know about the OP, but we quite often rent cars for a weekend trip to see if we want to go out and buy that particular car, rather than just having a short test drive from the dealership. So if I want to test drive a Subaru Outback, don't bother trying to give me some sort of Chevrolet, or even a Subaru Forester; I need an Outback, and no other vehicle, regardless of size, will suffice. It does restrict our choices to renting from the dealer (some do), or the larger rental companies, but we get what we need.