The three types of goods Trent Hamm at The Simple Dollar tends to impulse buy online are books, board games and video games from Steam. To help cut down on this tendency he keeps three sticky notes on his computer monitor that list the books and games he already has but has not yet read or played. Whenever he's tempted to buy another one of these items, usually a glance at what he already has on tap and hasn't used yet keeps him from making the impulse purchase.
Whether your impulse shopping weaknesses are Lego sets or vinyl albums, using this technique should help you control your purchases, particularly if you're not a collector but just an enthusiast.
My Top Tactics for Reducing Online Shopping [The Simple Dollar]