After tax dollars into a traditional IRA

thoughts on contributing to a traditional IRA with after tax dollars if you make too much money to contribute pre-tax dollars or to a ROTH IRA. Is it worthwhile? Is it difficult track the after tax contributions so that you are NOT double taxed upon withdrawal of those dollars? Can the pre and after tax dollars co-mingle in the same IRA or should you open a new IRA to keep it separate if you choose to contribute?