The best way to ensure your Will is valid, is to use the help of a professional Will writer. Solicitors and Will drafters have a duty of care to ensure your Will is valid. They also have indemnity insurance which covers your estate if a claim is made against it because of their negligence. They have a duty to ensure the two main areas which are necessary for a Will to be seen as valid, i.e. the capacity of the testator and the formal requirements involving the witnessing of the Will have been carried out correctly.
Firstly, any Will you make will not be valid if you are not regarded as having the capacity to make it. There are 2 aspects to this, namely age and mental capacity and they are linked.
Unless you are in the Armed Forces, you are only regarded as having the mental capacity to make a Will after the age of 18. Where you are 18 or over, you must understand the extent of your property/estate and be aware of the people who you would usually be expected to provide for (even if you choose not to). You must also be free from any delusion of the mind that would cause you reason not to benefit those people. These principals were first established in a very old case called Banks v Goodfellow in 1870. The judgement in this case used alongside the Mental Capacity Act 2005, still holds fast today. They will also ensure they have considered the "Golden rule" (Re Simpson [1977]). This states that however straightforward the will and however tactless the suggestion, ‘the making of a will by an aged or seriously ill testator ought to be witnessed or approved by a medical practitioner who has satisfied himself of the capacity and understanding of the testator, and records and preserves his examination or findings.’
Secondly, there are the formal requirements of the Will. The general requirements are that it must be in writing and signed in the presence of 2 witnesses, who cannot be beneficiaries under the Will and must also sign it at the same time.
Sounds simple, doesn't it? But you would be amazed how many people get it wrong.