what do I do about slow paying clients?

[Martin]

I have a small business and slow payments is starting to cripple my cash flow. A lot of my clients seem think its okay to pay on 60 days. I recently requested payment for an invoice that went out in July with 30 days. The client has informed me he sent a check, then when the check didnt turn up I was informed that he would pay via bank transfer. I have waited three to four days and nothing has shown up. I have phoned only to be informed that the money left his bank but this has never hit my account. I am in the process of sending a solicitors letter....or do I wait.