Regarding the "yearly" cleaning, while some items are dependent on gallon usage, the oil filter at the tank should be changed yearly, as the sludge can build up even with no flow (sludge is created by algae, and will continue to grow under some circumstances). Oil filters are cheap.
If you are reasonably handy, replace the old-type filter with a Gar-Ber spin-on filter head. Easier to change, and much better filtration. If you want to get fancy, keep the old filter and add the spin-on after it, for double filtration (old filter at tank, spin-on at heating appliance). If you do that, get the better Gar-Ber with the vacuum gauge and the mounting bracket; the bracket simplifies installation, and the gauge will let you know when to change the filters, rather than relying on time - just watch it with the system running.
(Gar-Ber is one particular brand, but Westwood also makes a quality filter)
The actual spin-on filters have a Fram equivalent, so you can get them locally, without going to the heating-supply store.
Be careful working on the fuel filter at the tank. If the nipple going into the tank snaps, you can have a very large mess on your hands. Always use a backing wrench if you have to apply any torque to the filter (they can get tight over time). Never use Teflon tape on fuel oil fittings. It's a very bad thing. The tape can release strands when the pipe threads cut into it, which can destroy your oil pump. Not to mention that it keeps the joint loose, which is great for disassembly, but we don't want that oil line to disassemble itself... Use Gasoila or other pipe dope that is designed for fuel oil usage.
Regarding the annual cleaning, there is a brush made for pin boilers. It's actually called a "Weil McLain brush" by most supply houses. It is a wire brush, and is flat, so it fits between the pins. Just be very careful near the back of the boiler, to protect the ceramic. Slide the brush down gently until you touch the top of the target, then mark it at that point, so you don't over-extend it. Bright spray paint works well, since it won't slide like a strip of tape.
You want to clean it with the boiler warm, after it has run a few cycles. Don't do it in the middle of summer, with the boiler shut down, as it may have condensation mixed in with the soot, which ends up being like concrete.
Also, clean the blower wheel in the burner, and make sure the air inlet slots (near the pump on Beckett and Carlin burners) are clean. Lint plugging up the air inlet is the leading cause of boilers running too rich and plugging with soot.
The biggest problem with cleaning it yourself is that nozzles aren't exact. They have as much as 10% variance in fuel flow. When you replace the nozzle on an oil burner, the combustion needs to be re-adjusted with instruments. It might be right on, by dumb luck, but it can be running significantly rich or lean, if that nozzle isn't close to what the last one was.
If you've already done the nozzle, the pump strainer, and the fuel filter, and brushed/vacuumed the heat exchanger, you can ask a service company to do a combustion test. Most companies I've dealt with have a set price they charge for coming out and doing that. If you tell them you want someone to check the combustion and look the system over for any possible repairs or improvements, they'll likely give you a better price in order to get a tech over there who may find something to sell you. If he makes a recommendation, think it over and see if it passes the BS test. It might be the "product of the week" from an unscrupulous company, or it may be an important repair/improvement that you should actually consider.
The second biggest problem is the electrode setting. They are pretty sensitive, and proper adjustment is necessary for clean and reliable combustion. While you're at the heating supply store, buy an electrode gauge. If you have a Carlin burner, there are two different gauges - one for the EZ-1, and one for the older (99, 98, etc.) models. Tell them which model you have, so you can get the right one. If you have a Beckett burner, get their Z-gauge, as it will check the head distance and the nozzle gap, with one gauge. These gauges are all plastic, and should not be used with a hot burner. I mention this because I've seen experiences service techs do it. Just because you had the drawer assembly out to replace the nozzle, and it has cooled down, does not mean that the burner end cone will be cool...
Joe