One simple way...

I shackle the jib blocks low to the forestay adjuster. Untie one sheet from a jib car and attach to the forestay (use a small 3/16 shackle around the forestay wire and tie the sheet to the shackle). Step the mast as normal. With your shoulder against the mast holding it up, bend down and sheet in on the jib sheet which holds the forestay at the adjuster. You could place the boats nose downhill to help. All this can keep the mast upright while you move forward to pin the forestay. Another option, tie a line from the main halyard shackle to the mast stand on your trailer. After the mast is up, hoist the halyard and tie of when taunt to the trailer mast stand... holding the mast up.