Use ALT+W/S to set trim, and try not to use ASAS on spaceplanes. Avionics is best (usually) unless you're sending up Arkbird-type planes (Ace Combat 5, I went there). Also, try to keep all your fuel as centered on the aircraft as possible so that as it drains it doesn't shift your CoM much. Aside from that, angle your wings by 5 degrees. Without mods, wings with zero AoA generate no lift.