What kind of browning solution are you using?
In general, the first thing to do is be sure the metal is clean and prepared. That is, degrease and sand/polish for the finish you want. Browning solutions work best if you don't sand too fine. I like to use a 320 wet as the final grit. 600 is too fine - you'll have a very smooth surface but it will take a lot of work and several coats to get it even. 220 as the final grit gives a nice surface for an antique look. Use latex gloves for handling after the final degrease.
Think ahead about how you're going to hold the parts - you don't want to touch them while they're wet with solution or curing. I usually build a wood fixture from scrap or dowels that's attached to a sawhorse; slide a barrel onto the dowel, hang small parts from the wood frame, etc.
Hot brown solution, such as Birchwood Casey's Plum Brown: the key is temperature control. I use a propane torch and heat the metal slowly and evenly. It must be hot enough to quickly boil away a drop of water but not so hot as to change color. Don't be surprised to see moisture come up out of the pores - that must all be boiled out. Use a large swab to apply the solution so you aren't tempted to rub it on. The softer you apply it the better. Let the heat do the work. Wash thoroughly in hot water and rub off the scale after it's cooled down.
Cold brown solution, such as Laurel Mountain: the key is humidity, the more the better. I place the fixture in a bathroom and turn on the hot water in the shower and close the door; when it fogs my glasses I start applying the solution. Others build a 'sweat box' with a tray or wet towel in the bottom and suspend the parts in that. Cold brown looks terrible after the first coat and you'll think you've ruined the parts - just keep applying additional coats and you'll be surprised at the result.
