Is there a general guideline for how to parlay a lead in development and a slight positional advantage into a definitive win? I'm playing a game now online, and I'm stuck for a way to keep and exploit the upper hand. I get stuck like this a lot, and I figure it must be for lack of principles or tactical vision.
Converting advantages such as a lead in development or a better position are practices that operate on a higher level (conceptually, at least) than tactics. The most common forms of tactics are those than result in the gain of material or a mating net, much more tangible results than say a weak square or an initiative. Pure calculation and a sharp tactical sense are important aspects of chess, but for the majority of the game they will be largely separate from positional play. Therefore, I would say that this is more supportive of your statement that it is a lack of understanding of principles rather than a weak tactical vision.
Let's address the specific examples that you outlined in your question. First, what does a lead in development entail? There is one main principle that you can utilize - initiative. The initiative is dictating the flow of the game. You can force your opponent to respond to your moves, allowing you to execute whatever plan you want while placing his own goals on hold. In such, it is common for positions where one side has the initiative for that player to launch an attack, given that the opponent is not adequately prepared.
If you can elaborate on the "slight positional advantage" (Minor piece difference? Pawn structure [backwards, isolated, doubled, hanging, passed]? Open files? Weak squares? Space? etc.) that you mentioned in the question, I can recommend other plans that are associated with the appropriate imbalances.
Several, distinct kinds of advantage will arise during chess play. These include among others
- a lead in development and/or the benefit of a tempo,
- heavier influence over the center or over a key square,
- greater space and/or room to maneuver,
- the coordination of pieces,
- sounder pawn structure,
- a better bishop,
- protection and/or overprotection,
- a safer king,
- the control of an open file,
- an advanced knight,
- an extra pawn,
- a passed pawn, and
- an extra piece or the exchange of a minor piece for a rook.
As you know, a player values such advantages insofar as he can convert them into either (a) a mating attack or (b) a won endgame. However, your question is: how does a player convert?
No one answer exists, of course, but there are indeed principles one can bring to bear. Note the order in which I have listed the several advantages. Roughly, the advantages toward the top of the list will tend to arise earlier in the game and/or to last the shortest. Likewise roughly, the advantages toward the bottom of the list will tend to arise later in the game and/or to last the longest. A player would like to convert any advantage directly to a mating attack or a won endgame if he could, but often he cannot convert directly. What he can do is to convert one advantage to another, generally (though not always) trading advantages down the list.
For example, a lead in development might afford you a chance to cramp your opponent's maneuvering, leaving the opponent to free his maneuvering at the expense of pawn structure or of a good bishop. You might compromise your king's safety to control an open file, and you would probably trade any of these advantages to gain the advantage of an extra piece.
Most of the list's advantages can be converted directly to a potential mating attack under the right condition. Of course, when the condition arises, you can stop trading down the list and press the mate—which either wins you the game immediately or, one hopes, at least converts to a won endgame. However, the right condition will arise only in about half of your games. In the other half, you trade advantage for advantage until the middlegame has run its course, then apply your endgame skill to convert the advantage to a win.
But, how should you decide when to trade advantages?
The answer is that one trades advantages when the chance to do so arises and either (i) the advantage to be gained is stronger and/or more permanent or (ii) the advantage to be lost is doomed to vanish in any event. For example, a lead in development and/or the benefit of a tempo is usually doomed to vanish, fast. You cannot keep it, so you had better trade it.
You have not mentioned any specifics of your games, so I cannot tell for sure, but my guess is that you may not be trading your lead in development for another advantage fast enough. The general rule is this: develop and bring to bear as many pieces (not pawns) as you can in as few moves as possible; then, without wasting a single turn, immediately press the attack. Your opponent must react, and his reaction is likely to present you the chance to seize another advantage.
Do not attack until all your available pieces are in the fight, then attack without delay. When your opponent reacts, seize what you can. Many exceptions exist, but this is the general idea.
See what you cannot do with such advice. Good luck.