The negotiations leading to the execution of a pre-nuptial agreement is indeed a delicate affair. Care must be had to not permit overreaching, or any sign of trying to pull the wool over the other party's eyes, lest the nuptials may never occur.
In order for a pre-nuptial to resist attack (and there is almost a certainty that in the event the marriage ultimately leads to divorce the agreement will be subject to attack by one of the combatants), full disclosure is the key. All assets should be disclosed and appended to the agreement as an exhibit in order to foreclose the argument that there was not full disclosure of all financial data of a party.
When estimating the value of assets it is normally a good idea to estimate on the higher value so there can be no claim of hiding true value. Once full disclosure has been made, and each side has independent attorneys to represent them, there is little chance that the agreement will be set aside by the courts.
Of course traditional grounds of fraud, mistake of fact, or unconscionability may nevertheless be employed if they otherwise exist.
The hope of all parties concerned is that the marriage will take place and the couple live happily ever after. Negotiating a fair agreement is a right step in this direction.