I know that using things like Dawnbreaker, which results in messy explosion sending items flying everywhere and ash piles, makes your save file size rack up quick. Are there any other items / methods / whatever that should be avoided to keep your save size down?

I'm asking because I'm beginning to notice frequent lag every half an hour or so, and want to fight it as much as I can.

I would suggest simply turning off the Auto-save feature. If you are getting the lag about every 30 min or so, then the setting for auto-save might be at 30 min intervals. Turn it off and the lag will go away. Just be sure to save frequently, like say before going into a battle or high risk areas.

THiS is if you are on a PS3 though.

Also, Auto-save can create like 3-4 separate files of it's own... and that is on top of say the 1-2 manual saves you might have.
Make a save file manually, note that it is actually there and saved.... and delete all other save files. Start fresh with that last save you did. SHould result in noticeable changes. But remember again.... you HAVE to make manual saves on your own... or might find yourself about 3 days back for example than where you are presently.

You don't have to turn off all Auto-Saves.

Wait and Rest Auto-Saves should be left on (1hour wait is easy way for quick save)
Travel Auto-Save can freeze up on larger saves. (or just drag out load times)

Also depends on amount of play time you have(Plus file size). Corpses often go poof after leave cell or area, inless it has been patched.
after 10-30days stuff is suppose to reset.
When you reenter a cell you have been in (after the alloted time) It'll reset as you enter. So you often see more lag more you play.

Arrows and Weapons (Shields to) from stuff you kill do not reset with corpse (Its a bug... not sure what cuases it, but after sometime you'll notice weapons and stuff drop by bad guys will be on ground and corspe missing.
Can be solved by picking up weapons and shields as you clear out caves, area's etc. Placing them back on corpse.
Other items laying on tables and stuff don't matter, as it will reset.

I havn't had really any issues with Dawnstar,

I ussally make a save (as back up) then rest a few days then save again as my main. You'll often find save file shrinking 100-400kb sometimes. (or get bigger by 100-200kb)
11-12mb is about were i notice abit of lagging switching cells, and longer load times. I ussally use PS button to quit game every 1-2hours and restart game. Everything tends to go back to normal.