Many years ago I asked my bank to stop addressing correspondence to me as 'Raedwald Esq.' and use 'Mr' instead; the only effect of this was to spur the bank into the solecism of addressing me as Mr Raedwald Esq. During the '90s 'Esq' disappeared altogether as institutions realised that as there was no female equivalent, it could be considered sexist to continue to use it. Realising this now has caused me to change my mind; I think it's high time we reclaimed Esq.
So, if you've held a commission in a rank equivalent of army Captain or above, hold a Master of Arts degree, are a Barrister (but not a solicitor unless a Bachelor of Law), a JP or a holder of office under the Crown, or the son of a knight or petty noble or a Physician then reclaim Esq as your own. If you're a bloke.