Besides having no didactic support in scripture, I simply detest the sinners prayer. That said, I do believe prayer can play a helpful and encouraging part of presenting the gospel and after calling on the individual to repent and believe. First, the person sharing the gospel should pray. Pray that God will reveal himself to the individual you are sharing with. Ask God to give them understanding of the scriptures and to display to them his mercy and grace. Encourage them to begin a lifetime of prayer immediately. Basically I would instruct them and encouraging them to go the Father in prayer in the name of the Son. At no time would I ask them to verbalize their commitment to Christ in any way that could be misconstrued as a sinners prayer. I do not want hope placed in a prayer. Even if you provide a detailed disclaimer that saying this prayer in no way saves, some individuals will still view efficacy in the prayer itself. If they give a positive affirmation that they believe the gospel then I would do my utmost to get them into church so they can feast upon God through his word, sacrament, prayer and fellowship.
