With RGB strip
Mounting on conduit
One method of achieving a straight line with RGB strip is to zip tie it onto electrical conduit. Conduit is cheap and easily sourced from hardware stores. Repairing or replacing strips requires cutting off the zip ties.
Mounting in Endcap
Another method for making straight lines with RGB strip is to buy Clear Endcap, which is found at Bunnings in the Polycarbonate roofing section. Endcaps come in 3 metre lengths but can be shortened with a hacksaw or snip tool. Endcap is not as cheap as conduit and at times you may need to visit several Bunning stores for larger quantities. The Endcap approach looks clean and neat during the day and is UV treated - something to consider if you intend on leaving them installed year-around around windows or other areas.
With RGB strings