"But which Division should my designs be for?" you ask. Again, you don't know. You should have at least one design in each division, and a few extra in Division 15. Division 15 is a price limit where you can build a decent car, but it still presents a challenge. If you are really serious about your duelling, you should have several designs in each division. Have a variety of cars to choose from, such as a fast car with good HC for a racetrack arena, or a car with extra ammunition for scenarios with many participants, etc. Playtest your designs before the con to find their strengths and weaknesses. This may sound like a lot of work and bother, but to those who really want to win it won't be that hard. And it will be worth it when you watch your adversaries scramble to work up a design a few minutes before the round.
When you are designing your vehicles, be sure to make them ready for anything. Often when you only play with certain friends, your vehicle designs will be styled to fit how they play. For example, if your friends never shoot at turrets, you put very little armour on top. At the con, though, your friends aren't the only ones who will be there, and you may run across some character who only shoots at turrets. You should always carry a fire extinguisher or fireproof armour, because you never know if someone might be using flamethrowers or lasers until combat starts, and by then it is too late. Bring enough vehicle design material and sheets in order to be ready for any scenario that may be thrust upon you, especially in the later rounds.
If you are carrying hand weapons, don't forget about them. A well-aimed blast from an SMG, grenade or even light pistol can save the day when your vehicle weapons are out, or can't hit.
If you are driving a compact or sub-compact car, don't forget the additional -1 for firing at the body of the car. This modifier is often forgotten but it can save a lot of wear and tear on a smaller duellist.
Making deals or agreements with other players may or may not be allowed. Ask the referee if you can make any deals before you do. Also remember that there is no guarantee that your "partner" won't plug a shot into you when given the chance, or vice versa.
Unfortunately, there are a few dishonest duellists out there, and/or some who don't design correctly or keep proper records. During combat, keep track of everyone's shots and damage sustained. If you suspect someone of cheating, ask the referee to check it out, and don't get prematurely angry at anyone. Cheating and/or making mistakes during vehicle design is especially easy. At a well run tournament, the referee will check all designs before combat. If he doesn't, pay attention to your opponent's car during combat. if it has enormous amounts of armour coupled with heavy firepower and fast acceleration, you can bet there is something fishy with the design.
Your referee is the one who determines what is right and settles questionable situations. Before the round starts, ask any questions that you may have, such as if you can use weapons from ADQ, or if you can hide mines in smoke. Many of your favourite tricks you've developed in your home group may be illegal on other players' groups. If you have any doubts, it's better to ask the referee in private before the action starts than to get into an argument in the middle of the game. And be sure to carry the rulebooks with you, so you can quickly check things out for yourself.
Good luck, and may you always make your control roll!