Such heartfelt words of warning come from headhunters in the City of London who have repeatedly witnessed the extent to which angst and recrimination can surround a resignation.
It is, of course, in the headhunter's interest to ensure that you do as you set out to. Jonathan Evans, managing director of Sammons Associates, a dedicated financial services search practice, says: 'You can have the right candidate for a job and then everything falls apart when he or she goes in to resign. It is the point at which all deals have the potential to come apart.'
While it is ultimately the candidate's decision whether to stay or go, the role of the headhunter is of prime importance in taking him or her through the resignation process, says Evans.
Whether or not you use a headhunter to hold your hand, you need to work out for yourself and be very clear about the various reasons for your decision. It might be useful to make a list, one that you can run through with a partner just to check on how sensible it all seems. You need to think back to how you got to this position in the first place, and why you are exploring something new. Crucially, you need to be mentally prepared for any tactics your employer decides to use to argue against your decision.
Horror stories abound in the investment banking industry of people being locked into rooms and refused access to the outside world while they are 'persuaded' to change their minds about their resignation. Intimidation and guilt are just two of the tactics employers may resort to when businesses are at stake as a result of the threatened departure of a key individual. But half the battle lies in being prepared for anything your employer may throw at you, and being clear in your own mind about the reasons for your decision.
Ben Worsley, managing director of headhunter Central Search, says: 'Be prepared for pressure and for your boss to get upset. They may try to make you feel guilty, but you will almost certainly feel that anyway, and you will probably also be scared. Work is a core part of our lives and walking away is always a risk.'
Worsley argues that once you start the process, you must do it right by being straightforward and honest about your reasons for leaving. Do not let your boss find out through other channels.
Too much honesty is not necessarily a good thing either. Anne Semple of SHP Associates says: 'Most firms use exit interviews on resignation and there is a temptation, at such an emotional time, to slag people off. But the City is a very small place. That person you were rude about could be your next boss.'
Paul Wilson, managing director of Michael Page Human Resources, says: 'If there is a degree of edge and you are going to a competitor, you might be better off not saying where you are going. But if you are going to tell all and you can't resist inflating your future salary, you could be in big trouble when the bank you are leaving and the one you are joining end up merging.'
Both massaging the truth and saying more than you need are best avoided if you are serious about leaving. The important questions you need to answer for yourself are whether the job fits in with your other goals, whether the potential new employer is committed to the sector and whether all the factors add up to making this resignation the right thing to do. You must also be clear to what extent it is a question of money, because that is the first fence at which resignations falter.
At junior levels the employer often reacts to a resignation with a counter-offer that does not amount to a substantial rise, but is tempting. If your fundamental reasons for resigning have not changed, it may not be wise to accept such an offer. Counter-offers are widely judged only to paper over the cracks in an unhappy circumstance till they reappear.
At senior levels, counter-offers are often made to keep the person resigning in the short-term, with the money promised in the counter-offer coming in reality from future bonuses or future promotions.
Ben De Haldevang at Richmond & Co says: 'It is important to make your resignation letter brief and to the point, and to make sure your employer can find a good, positive reason in it to accept the resignation.'
On a more cynical note, although bonus payments agreed with the employer and technically paid into your account by the banks' BACS clearing system are not subject to alteration, funds that may not have cleared your account before you resign may be stopped.
If you are at all senior, make sure you consult a lawyer first to make certain you are not in breach of your contract in any way, says one veteran headhunter.
Relationships between employers and employees are generally deemed to have become more professional since the days of the late 1980s. Then company cars were often found left in inappropriate places with the keys in the ignition as a gesture of defiance or pique after a resignation.
But despite greater efforts within many an HR department to ensure good working relationships, there is still little love or loyalty lost between those determined to head out of the door in the increasingly cut-throat world of financial services and those trying to build a business.