While a decision will have to be taken on a case-by-case basis, in the following circumstances it would be advisable to make use of trade secret protection:
When the secret is not patentable.
When the likelihood is high that the information can be kept secret for a considerable period of time. If the secret information consists of a patentable invention, trade secret protection would only be convenient if the secret can be kept confidential for over 20 years (period of protection of a patent) and if others are not likely to come up with the same invention in a legitimate way.
When the trade secret is not considered to be of such great value to be deemed worth a patent (though a utility model may be a good alternative in countries where utility model protection exists).
When the secret relates to a manufacturing process rather than to a product, as products would be more likely to be reverse engineered.
When you have applied for a patent and are waiting for the patent to be granted.
It is important to bear in mind, however, that trade secret protection is generally weak in most countries, that the conditions for, and scope of, its protection may vary significantly from country to country depending on the existing statutory mechanisms and case law, and that the courts may require very significant and possibly costly efforts to preserve secrecy. Patent or utility model protection, wherever possible, will provide much stronger protection.
"Trade Secrets are Gold Nuggets: Protect Them" [PDF]