A declaration is a statement made by the importer or the exporter, and the responsibility for it stays with the party that signs it, even when a third party prepared the papers. That single fact explains most of the problems in this area. The file can be assembled perfectly and still be wrong, because it was assembled from details that were never checked against the goods, the order or each other.
The most common problem is a description that says almost nothing. Parts, samples, gift, accessories and machinery with no further words give a reader no way to test the classification, and they invite the question that holds the file. A description should name what the item is, what it is made of and what it is used for, in the words a person who does not know your product could check. The description and the classification have to agree with each other, and both have to agree with the goods a person would find if the carton were opened. Wording chosen to make a consignment look unimportant does not change which rules apply to it; a sample that is sold later, or a gift that has a value on the invoice, is still declared as what it is.
The second is value. A declared value that is lower than the transaction, or split across two invoices to keep each one under a threshold, does not remove the rules that apply to the goods; it moves the risk onto the party that signed the declaration. Where goods move free of charge, they still have a value for declaration purposes, and freight and insurance have to be treated consistently with the terms of sale. An entry can be queried long after the goods have been released, and a correction made months later costs more time and more explanation than the same correction made before submission.
The third is the set itself. Weights and quantities that differ between the invoice and the packing list, carton counts that do not add up, a shipper or consignee name spelled differently on the transport document, a currency that is not stated, an invoice number that does not appear on the documents that refer to it, an origin statement that no one can support. Each of these is small, and each one is a reason for a question. Translations matter too: the description on the commercial invoice should be readable by the office that will look at it, and anything signed by an authorised person should be signed by someone who is actually authorised to sign it.
Classification and origin deserve their own file. Record the code you entered and the reasoning behind it, with the material, the function and the rule you applied, so that a question asked later can be answered from the record rather than reconstructed from memory. That habit also makes the next shipment faster, because the questions have already been answered once. Origin rules are not the same in every destination and turn on where the goods were produced and what was done to them. Duties and taxes depend on the classification and origin rules that apply to your goods, and the decision on an entry rests with the customs office that receives it.