We all know about how Lord Vishnu helped Lord Shiva and vice-versa. Here is a story which indirectly points out how Lord Shiva helped human form of Lord Vishnu- Sri Rama.
Once Narad asked Vindhya mountain (in Madhya Pradesh) to raise his height to get the attention from world and to get the title of highest mountain. Vindhya mountain raised his height which separated north from south.
During the marriage of Lord Shiva and Parvati, there was a huge crowd in Mt.Kailash. Due to over-crowd with all deities in one side, there was an imbalance in earth. So Lord Shiva asked Sage Agastiyar to move to south in order to balance the earth. Everyone was surprised to see Lord Shiva asking a sage of 4 feet tall to balance the earth.
Sage Agastiyar understood Lord Shiva and he moved with his family to the south. On seeing sage Agastiyar, Vindhya greeted sage by bowing. Sage told Vindhya that he was moving to south with his family for an important cause and asked him to be in the same state till he rushes back.
Vindhya promised the sage and remained in a bowing position as sage never returned back. When sage reached the other side (south) the imbalance of the earth stopped.
In a way, Lord Shiva already created the path for Rama to move down the south by sending sage Agastiyar before itself. He helped Rama in Rameswaram just before the construction of Setu Bridge.
So Lord Shiva helped Rama to get his wife Sita back by controlling the mountain and ocean.
Note: Story of sage Agastiyar curbs Vindhya mountain can be interpreted in a way to see how it helped Rama. This is my POV and I welcome the opposite views.
The following lines are from Valmiki Ramayan
Ref: http://www.greenmesg.org/bharatavarsha/puranic_events/rama_events.php
Sri Rama revisits the hermitage of sage Suteekshna and asks about the hermitage of sage Agastya: During forest exile, Sri Rama stays in various hermitages and ten years gradually elapses. Thereafter Sri Rama revisits the hermitage of sage Suteekshna. After staying there for sometime, Sri Rama asks about the hermitage of sage Agastya. Sage Suteekshna tells about the hermitage of the brother of sage Agastya four yojanas from there on a plateau with many lakes. From there the hermitage of sage Agastya is one yojana away. (Valmiki Ramayana 3.11)
Sri Rama tells about the story of how Agastya terminated the demons Vatapi and Ilvala:
Sri Rama while going to sage Agastya's hermitage halts near the hermitage of the brother of sage Agastya. There Sri Rama tells about the story of how Agastya terminated the demons Vatapi and Ilvala. (Valmiki Ramayana 3.11)
Sri Rama halts near the hermitage of sage Agastya and narrates various glorious deeds of the sage:
Rama narrates various glorious deeds of the sage to Lakshmana. This includes the incident where the sage curbs the Vindhya mountain. (Valmiki Ramayana 3.11)
Sri Rama Rama visits the hermitage of sage Agastya near Panchavati
During forest exile Sri Rama visits the hermitage of sage Agastya and meets the sage.
Sage Agastya confers various divine weapons to Sri Rama. (Valmiki Ramayana 3.12)
Sage Agastya tells about Panchavati forest two yojanas from there on the bank of river Godavari.
(Valmiki Ramayana 3.13)