Being a novice I decided to start some flowering plants in my windowsill. They are going better than I anticipated but some of the taller plants are about 4" tall and drooping. I dare not plant them outside as there is still a probability of frost. Can I use something such as a lolly stick or short cane to keep them upright?
Support yes, ice cream sticks are good (and yummy), straws stc.
And move them to a slightly cooler place - ideally somewhere out of direct sunlight thats not heated, but also not open to the elements, ie a well lit garage, or shady porch.
I assume your watering them, small regular amounts.
They may be too warm or have too little light. Hardy plants like lupins are probably better off in a cold frame.
Are they in trays?
They may want prickng out into individual pots and kept cooler around 40°F will do.
http://www.thegardenersalmanac.co.uk/Data/Lupin/Lupins.htm (http://www.thegardenersalmanac.co.uk/Data/Lupin/Lupins.htm)