This tool could be your friend if you want to continue doing it yourself. Always make sure you are turning the correct way.

Remove the seatpost and get something down that tube to the bottom bracket that will actually help remove the rust. Maybe try some phosphoric/citric acid with baking soda and water and that could help out. You could also try some heat but I would make sure you do it in a place with good ventilation because everything spayed or poured or whatever in those tubes will heat up and you don't want to breath it in. Generally I find WD-40 to be good for displacing water (hence the WD) but for lubrication I will look elsewhere unless it is a different product from the WD-40 company.

Before you put a tool on anything clean the area well and dry it, you don't want oil, grease or anything that will make things slippery on your tools. Use a good tool with good leverage that will not slip along with the B.B. Socket Holder putting the tool in a good sturdy vise and using the frame for leverage is a good idea from FBinNY You could also make one as others have suggested but I have found the Pedro's tool to be quite handy and easy to use.