I usually just come up behind them and come to a slow jog until they hear me... maybe breath hard just a little bit so they hear me.... and then they usually get the point and move over.
I don't usually say anything verbally because I'm not a real big talker and I feel somewhat like I am imposing by asking to pass.
I also figure that certain things in life don't really require words, and they should just naturally work themselves out, because people should just see the situation and respond appropriately. Having to even use words to communicate something so basic, seems wholly unnecessary to me.
Here's my little theory on the matter... just a random collection of brief thoughts.
(1) What we are dealing with here is an extension of a social norm. It's an informal customary way of dealing with people. Some social norms are complex, whereas others are so seemingly simply they almost don't seem like social norms. People tend to think of the latter as being "common sense," but rarely is it that simple.
(2) Because it is a type of social norm which is voluntary, there is no party who is "in the right" or "in the wrong," if one of both of the parties:
(a) do not understand the norm, because they have not really been exposed to it, and
(b) who did not consensually agree to it, and even moreso maybe they don't think it's a sound rule to follow.
(3) Expectation that others should abide by these social norms are to be judged 'in context,' particularly in regard to (2)(a) and 2(b), but also factoring in how complex the norm is. What we are dealing with here, as it pertains to the original question, is arguably one of the simpler norms, so that also has to be taken into account.
For complex norms, it is an unreasonable expectation of people to understand them if they have not been exposed to the rules such that they are familiar with them.
For simple rules, however, one could make the argument that there is a reasonable expectation that even those unfamiliar with the norm should understand them, as they are sufficiently obvious in nature such that anyone with sufficient mental faculties should be able to figure it out. However, one might argue that that fact alone does not mean they are ethically bound to follow the norm, just because they understand it. Point being, you can't get all pissy when somebody doesn't play the way you want them to play.
(4) The passing [hikers] from behind rule in trail running is a lot like the issue which runners face when they sometimes to to the running track to workout. If you ever go to a frequently used high school running track, you have likely run into this problem of having to pass assorted moo moos and middle aged house wives doing their little power walks, and slow walks on the inside lane of the track.
Pedestrians and slow polks who use the inside lane at the running track and don't want to make way for the 4 and 5 minute/mile guys tends to annoy the latter group greatly. And I won't like and say its never annoyed me, because it does irritate me, and it still does. But I ask myself are these slow polks "wrong" for hogging the inside lane of the track, and not moving over, and my answer is usually "no." It takes a pretty contorted argument to convince onself that they have some kind of "right" to exclusive use of the inside lane of a track thats not even theirs to begin with, and where no rules are laid out at the gate for all to see.
With regard to the norm on the track, first off, I can't really fault the moo moos and house wives too much for hogging the inside lane and obstructing fast runners doing their interval workouts, because maybe nobody taught them track etiquette. However, this norm is so simple, that one might argue that it is self evident that they should be able to figure it out for themselves, which is a plausible argument, but that doesn't negate the fact that unless a posted sign or agreement somehow exists binding others to follow a certain norm in order to use the facilities, then regardless of how much of a nuisance it is, they really don't have the follow the rule. It's annoying and inconvenient, but such is life. They ostensibly have just as much right to be there as you do.
(5) With regard to what social norm rule I think should exist, I would favor a rule that gives priority to the faster party, such that the slower party should make way for the faster party. I don't think it should require any special prompting on the part of the faster party to request that the slower party make way. I think the slower party should simply move over, and no words need be exchanged.
All parties ostensibly have a right to be there on the commons with everyone else, but I don't think in pursuance to that, that any party should obstruct anyone else in the process, and I would say that that is the basis for a rule which favors the faster party, and puts an obligational burden on the slower party to yield.
So... I would say the obligation is thus placed on the slower party, and the faster party coming up from behind should not be obligated to ask permission to pass. This of course assumes that the slower party is aware of the approaching [faster] party approaching him.
To the extent that I think the faster party coming up from behind should make a verbal cue or communication that they want to pass, I think that has more to do with the fact that sometimes (oftentimes) the slower party up ahead just does not hear an advancing party for a variety of reasons ("in the zone", hearing your own rapid breathing which drowns out the sounds of the upcoming parties, etc...), and so they cannot be faulted for that which they cannot hear.
But, since these social norms are voluntary.... though I favor this rule I laid out above, I do not see an ethical expectation that anyone is obliged to follow it, unless they previously agreed to follow it. Hence, I really can't get pissy when the forward party doesn't do exactly as I would like them to do, unless they are in some way intentionally obstructing my right of passage, which I think has more to do with a larger universal ethical rule which falls outside the scope of the much more narrow in scope social norm rule of rules of passing.