In the past I have polished well and sprayed with a clear laquer. You need a couple of thin coats at least, or otherwise you get specks/blotches of tarnish where the spray has missed.
"Renaissance" micro-crystalline wax from Picreator Enterprises Ltd, www.picreator.co.uk, tel: 020 8202 8972. Developed by the British Museum for conservation of paintings, metals, furniture etc.
Clear car lacquer works very well. If the brass object is outdoors, you can then recoat it thinly with an extra layer of Polyurethane varnish. Do not coat brass objects directly with PU varnish however, otherwise they will quickly turn several shades of green .
Personally I have had perfectly Good results with just single pack polyurethane varnish. There used to be a product on the market called "Antarnish" but I do not know if it is still available. Traditionally brass lacquer was just shellac but it may not be suitable for marine use.
What ever you use clean the items to be lacquered thoroughly and polish with Brasso or similar but then wash very thoroughly to remove all traces of the Brasso (washing up liquid and warm water) dry and handle with cotton gloves to avoid greasy finger prints!
Indeed, you can have good results with just PU varnish. But it depends on the quality, since some contain acid residues from the chemical process making them. But that may be not the case with recent products. Rather than polishing with Brasso, I found that slightly dulling the object first with a fine synthetic 3M pad provides a good key to the varnish, and prevents blistering to some extent. The gloss from the varnish makes up for the loss in brilliance. Still, the thinner the better.
I brought up my old brass compass beautifully by polishing with vinegar then tried the old P&O trick of covering (pretty thickly) with furniture polish. It lasted all season but didn't work so well on my oil lamp which is only brass plated, not solid brass.